Chapter Text
Growing up, most of Katsuki's friends assumed that he was asexual or something, as he had never shown interest in anyone and would completely mentally check out of the conversation once it turned into talking about relationships or celebrity crushes.
That wasn't necessarily the case though, you see, Katsuki had a very particular taste in men and women. One that made fantasizing about slim and fit people boring and pointless.
He was one hundred percent a fat admirer, no two ways about it.
Ever since he hit puberty, Katsuki had been browsing FA sites, gainer vlogs and fat fetish art blogs to get his rocks off.
Nothing made Katsuki come harder than jiggling bellies and curves that were rounder and more out of shape than they were last month.
There was a caveat however. It was less 'fat people' and more 'people getting fatter' that did it for him.
So relationships were pointless. He would never be satisfied just dating a chubby chick or fat dude and leaving it at that.
He would want them to get bigger.
Perpetually.
That's where Izuku came into play.
The two of them had a rocky past, sure, but for Katsuki it wasn't that he hated the guy, it was that Izuku's face happened to be attached to the growing bodies he'd fantasize about late at night.
This made interacting with the other boy highly uncomfortable as a tween, and with the anger issues Katsuki hadn't thought to address back then, he had mercilessly bullied Izuku.
It was only after years of therapy and being reintroduced to his childhood friend slash victim from a friend of a friend did they finally hit it off again.
Not to say that Katsuki no longer held toxic feelings towards Izuku, they had just… changed shape.
You see, Izuku was just so, so gullible. Naive and trusting, he only saw the good in people. Always believed in them, no matter how awful they were to him.
Katsuki and said boy's reignited friendship being example A.
Two years into their new, mended relationship, Izuku confessed to Katsuki.
Katsuki, if he was being completely honest with himself, had next to no interest in dating toned, gym rat Izuku. He was the opposite of Katsuki's fantasies.
But… what if?
He imagined for a second, a single second as Izuku stood there in front of him, waiting for a response to his confession, Izuku changing.
His mental image of Izuku's body suddenly expanded with fat, slowly at first, but speeding up. His friend's once bright and intelligent eyes hazing over with lust and greed.
It was there, at the edge of a friend's graduation party that Katsuki said yes.
Katsuki didn't want to start off their relationship with shit like big macs, gainer shakes, and weigh ins, no matter how hot it would be.
No, he had to be subtle about it at first. Guide Izuku into it.
He might have been naive and trusting, but the guy was also whip smart and would sniff out Katsuki attempting to corrupt him a mile away if he wasn't careful.
So he started small.
And small meant an entire year of the most boring, mind-numbing sex Katsuki had ever had the displeasure of experiencing.
Izuku wasn't complaining, but it took everything in Katsuki to stay hard when rutting into his boyfriend's bony ass.
Imagine him fat, imagine him fat. Was Katsuki's mantra while attempting to ejaculate.
Luckily, Izuku was usually so fucked out by the end that he never noticed Katsuki's lack of interest in him physically.
Finally, after over a year of dating, Izuku moved into Katsuki's apartment.
He had been prepping for this moment for a long time now. He had made sure that his apartment had accommodations added, such as being on the first floor, handles along the walls, a walk in shower, a widened toilet, a massive California king bed and a bariatric scale that read up to 500 pounds.
Although, if everything went to plan, that scale would eventually be replaced by one with a cap of 1,000 pounds.
Yeah, Katsuki was a bit of an… extremist, when it came to his taste in fat people.
He wanted Izuku to grow and keep growing. Forever if they could help it. Because as soon as Izuku would plateau, Katsuki would lose interest.
So, with the apartment prepped, Izuku moved in and Katsuki began his plan.
First was showing Izuku, who was a bit of a health nut, articles on incorporating more saturated and trans fat into his diet for longer term energy. Then a study about 'bad' LDL cholesterol, and why it was actually good for you. Also, a news article or three about why it was recommended for the average young male to actually be aiming to eat 4,000 calories a day. What Izuku didn't know however, was that Katsuki had meticulously edited the elements of the pages to say what they did.
With this false info swimming in Izuku's brain, Katsuki started leaving magazines around about bulking.
Faster than he thought he would, Izuku started changing his diet and declared that he would start bulking for the recommended limit of four months before cutting.
Katsuki pointed out that the best athletes bulk for double that amount.
Izuku, as always caved.
The first month Katsuki supported and manipulated Izuku into eating nothing but junk food, claiming that not only would the extra calories help him bulk, but that the composition of the food matched the advice on the internet that he had shown him.
Izuku hesitated, but ultimately ended up eating the twinkie.
Next it was fried chicken. Katsuki explained that the lean protein plus breaded carbs were a great combo for building muscle, completely ignoring the fried part of the name.
Finally, Katsuki introduced Izuku to a foreign brand of protein powder with a, for the most part, unreadable label.
It was only after his trusting, naive Izuku finished gulping down his first taste of the mix that he asked the question. "So, how many calories is in this stuff exactly?"
Katsuki rolled his eyes. He'd taken up position of Izuku's diet planner a month ago after telling his boyfriend that he should focus on his gains and not the nitty gritty numbers like he usually did. "First of all, I told you that you don't need to worry about the number, just the results. But if you really need to know it's around 250 per drink."
Add another zero to that number and it would be more accurate.
Izuku licked his lips, chasing the creamy flavor. "That means I can have two of them to meet the minimum 500 extra I need per day."
"I told you not to worry about that shit. I've got it all handled, Brat."
And Katsuki really did. By month four, Izuku no longer needed scientific studies or magazines to encourage him to overeat or gravitate towards junk food. He just knew that Katsuki had his best interest in mind and would guide him in the right direction.
And guide him he did.
Katsuki had always been good at cooking, so cook he did. Izuku got six-ish meals a day. Breakfast, lunch, dinner, dessert, two snacks and a shake. All either full of or cooked with lard and butter, deep fried or drenched in sauces.
Katsuki watched as Izuku's eyes fluttered closed at the first bite of the day every day, humming in acknowledgment of the tasty food. He'd always complain by the end though that he was too full and that Katsuki's cooking was too yummy to waste a single bite.
"Well it's a good thing you're bulking then." Katsuki would say.
Weeks of eating nearly quadruple the calories that he should have been eventually caught up to Izuku, but Katsuki was prepared.
He monitored Izuku's weight on their fancy scale, programed to show a fake number to whoever stepped on it. The real number was sent directly to Katsuki's phone for him to check on weekly. He also paid close attention to when Izuku was getting dressed, to see how his clothes fit him.
Once Katsuki decided that Izuku was having to shimmy his widening ass one too many times to squeeze into his pants, Katsuki quickly and silently replaced them with a new pair, one size larger, when his boyfriend wasn't home.
With his clothes fitting now, no longer how large he was getting, it was difficult to really tell how big Izuku was getting visually. Katsuki could always take a look at the scale's app to tell the exact number, but as exciting as those numerical milestones were, it was a shame that Katsuki couldn't really see Izuku's progress visually.
So Katsuki made a pact with himself. Once Izuku hit 350, Katsuki would stop replacing his clothes. Finally, he would get to watch as Izuku outgrew his wardrobe, how his sheer mass dominated his waistbands and shrunk his shirts.
Month eight rolled around, Izuku's final month of bulking.
Despite what Izuku had said in the beginning, of wanting to cut after he bulked, Katsuki had a hard time imagining his plump little boyfriend loosing any mass whatsoever.
Izuku had taken to stuffing his face with gusto once his stomach began to stretch accommodatingly. Each meal was slightly bigger than the last and Izuku sucked down every last scrap obediently. He'd whine and hiccup at the end, but beyond the complaints and the "never again"s, there was now a gleam of greed in his eyes as he pressed carefully into his swollen stomach after each meal.
They both knew that Izuku would eagerly eat the same amount, probably more, next time and the time after that etc.
All of Izuku's shameless gluttony had to go somewhere though. Over the months, the gym rat had indeed grown fatter. First into a bit of a pot belly, then a full muscle chub. It was when Katsuki had manipulated him into skipping more and more workouts, encouraging him to fall into a more sedentary routine, that Izuku's hard-won muscle began to melt away.
Muscle chub became bear, bear became super chub. Izuku's body had grown soft and jiggly, with a belly that hung over his waistband and and ass that wobbled with every step. He had reached and passed 350 weeks ago, and his gut was starting to hang below the hems of his shirts, his upper arms being squeezed by his sleeves. His pants were getting harder to pull up past his thighs, leaving Izuku to have to suck in before buttoning them underneath his overhang.
And somehow, Izuku still had no idea what Katsuki was doing to him.
What he was technically doing to himself.
Katsuki was loving it. Sex had become less of a chore for him now that he could see his boyfriend's progress, see him gain in real time. Slamming his hips into Izuku's growing ass became better and better as it became bigger and bigger. More cushion, more stretch marks, more jiggle. Katsuki would grip the bulging sides of Izuku's belly, the other man too aroused to notice, and play with the expanding fat as he breathlessly fucked his gaining boyfriend. Each week he could see Izuku grow a little heavier, a little wider, a little more obese.
It was, ironically, the last day that Izuku was to bulk when he realized just how much he'd gained.
Izuku had been struggling to button up his favorite pair of jeans for about five minutes before he finally managed to suck in hard enough to latch the button. In his heaving relief, the seat of Izuku's abused pants suddenly split with a loud RIIIIIIIIIIP!
Izuku froze, his fat, heaving chest stilling it's panting.
He gingerly turned to look in the bedroom mirror, truly seeing himself for the first time.
An obese man stared back at him, round, soft and out of shape. Izuku pressed one chubby hand into his bulging side fat and his other cupped his swollen left breast. The man in the mirror did the same.
Just as Izuku wanted to burst into embarrassed tears, Katsuki walked into their bedroom, hair still a little damp from his recent shower.
"Kacchan!" Izuku cried out, accidentally squeezing his cupped flesh, leaving quite the sight for his confused boyfriend. "Wh-What- how? Why?!"
"Use your words, Nerd." Katsuki's eyes were glued to Izuku's ill fitting outfit. "What-how-why, what?"
Izuku made a frustrated noise before gesturing at his expanded body. "What-how-why, this! I-I'm huge!"
"So?"
"So!?" Izuku cried. "What do you mean 'so'?"
"I mean, so what? You wanted to bulk up and you did. Congrats." Katsuki tried to appear calm. If he pushed too hard, or not hard enough, Izuku would reject his growth entirely. He had to act smooth.
"I… I mean yeah? But not this much!" Izuku whined, returning to examining his body. He ran a hand over his wide hip while pinching his double chin with the other. "It'll take forever to cut like this!"
Oh good, he didn't suspect Katsuki had anything to do with this.
"I can help you diet." Katsuki walked behind Izuku, hugging his fat boyfriend from behind. His arms couldn't fully wrap around Izuku anymore and he tried not to squish into his huge body too obviously, but wanted to make a point. "But you do know that I don't care what you look like. Fat, buff, skinny. I don't give a shit."
Huge fucking lie.
"Really?" Izuku sniffled.
"Really." God, what a lie. "I just want you to be happy." And morbidly obese. "Hell, you could get so fat you'd be immobile and I'd still love you."
Izuku chuckled a little, noticing with dismay how his body jiggled lightly as he did so. "Well, let's not let it get that bad, eheheh." His meaty arm raised to scratch the back of his head and both men watched as Izuku's upper arm fat hung and swayed at the action.
"Do you really wanna still try to cut?" Katsuki pet circles into the wide expanse of Izuku's soft belly, without really meaning to.
Izuku ignored the way his lower region throbbed with arousal as his belly was rubbed. "Y-Yeah, I gotta lose this weight. I can't leave the house looking like this."
Good, Katsuki thought.
With one little squeeze, Katsuki backed away from Izuku before his huge ass could feel Katsuki's erection poking into it. "I can help you diet then, you know, do some meal prep."
Izuku turned around, facing away from the evidence of his actions in the mirror to look lovingly at his boyfriend. "Thanks, Kacchan!"