Izuku is starting to realise that it's something that happens slowly, then all at once.
At first, it's just that Katsuki can lift him, strong arms wrapped around his torso but he'd fall if his legs aren't wrapped around that slim waist. Then it's noticing the upped weights in the gym and how it's reaching Kirishima-levels of heavy. Because ever since their second year, Kirishima has been built like a tank and no one's ever managed to match him on the bench. Katsuki wasn't quite there during second year but third year he was getting close.
No, the real hit-him-for-six moment was when Katsuki had him pinned against a wall in some desolate building after a mission, solid arms caging him in and Katsuki's literal tree trunk thigh wedged between his own.
One For All still tingles across his skin and occasionally sparks green but either Katsuki doesn't notice or he doesn't care. He just cares about getting his mouth on Izuku's and hauling him up with one arm around the number one Hero's waist. It makes Izuku gasp and wrap his legs around the blond's (still small but not as small) waist but the little voice telling him it's no longer necessary to avoid a fall is something new.
Katsuki's gauntlets are on the floor, arms twice the size of Izuku's still caging him between his Hero partner and the concrete wall. And good lord does the feeling of Katsuki's dick rubbing against his own feel amazing . The blond's arms and legs are not the only things that got bigger since high school.
In no time, their Hero costumes are removed just enough and Katsuki's got two spit-slicked fingers in Izuku's ass.
The greenette is moaning whorishly, Katsuki's long, thick fingers compared to his own are heaven and reach all the right spots as they stretch him quickly. It's been a long while since they'd last had sex and Katsuki was wasting very little time. He adds a third finger and Izuku cries out, ungloved hands scrabbling for purchase against broad shoulders and thick hair.
God, everything about Katsuki was thick.
Izuku gets a good grip on the blond as he removes his fingers and lines up his cock, feeling the way his shoulders tense as he fights the urge to just bury himself in Izuku's ass. Izuku whimpers when the blunt head brushes against his hole, moans when Katsuki starts to press in and wails at the stretch as inch by inch, his ass swallows the blond's length.
It burns , even with the limited prep but it's the good sort of burn that Izuku quickly relates to the promise of good, solid fucking.
Which is exactly what Katsuki gives him.
The pace he sets is torturously slow at first, making sure Izuku can feel every inch of his cock dragging against his walls before slamming back in, to the hilt. The grip Katsuki has on Izuku doesn't leave much room to move, so all Izuku can do is take it, moaning and feeling his eyes roll into his head every time Katsuki hits his prostate with stupidly precise accuracy.
He knows he's scratching whatever exposed skin Katsuki has around his neck, biting down on the juncture of his shoulder but if the blond cares, he doesn't make any move to stop him, just keeps increasing his thrusts until Izuku's literally shuddering through his first orgasm, untouched.
It shouldn't be as hot as it is, coming only from his prostate being abused, but Katsuki moans, low and hot in his ear as he does and Izuku's gone .
His legs are losing their strength with the intensity of the aftershocks and he has a brief moment of panic that he'll fall but Katsuki's having no trouble holding his weight as he continues to chase his own release.
It's almost too much, feeling the consistent drag and slamming of Katsuki's cock inside him, smelling the blond's salty caramel scent and feeling the corded muscle holding him upright. He comes again when Katsuki lets out a particularly filthy grunt against the skin of his neck where he's busy sucking a hickey into freckled skin.
Izuku cries out with each hard thrust, over-sensitive and teetering on the edge of fucked out when Katsuki buries himself deep and lets go, pumping Izuku full of cum. Izuku whimpers at the sudden heat, moaning weakly when Katsuki pulls out and adjusts.
His feet touch the ground and instantly his knees give out but Katsuki's there to grab him before he eats dust.
The blond hauls him back up like he weighs nothing and Izuku just about swoons. He's definitely put some beef on since high school, a side effect from the Quirk and Hero training and it's been a long while since someone's lifted him that damn easily . He gives Katsuki a wobbly, fucked out, blissful smile and hears the blond huff affectionately.
Then there are gentle hands carefully putting them both back together enough to get home and Izuku practically floats into their apartment in a haze. Katsuki leaves him long enough to shove him towards a started bath and to put leftovers in to reheat. Izuku washes absently, mind still glued to the fact he hadn't properly noticed just how jacked Katsuki was until that moment he was pinned.
Katsuki finds him a few minutes later, staring blankly at the tiles and muttering at a mile a minute.
He kicks Izuku gently, rinsing green hair and herding him into the bath before grabbing the soap and washing himself. It's quiet for a moment until Izuku leans over the edge of the tub.
"Hey, Kacchan. How did you get so big without me realising?"
"Does it matter, nerd?"
Izuku shrugs and rests back in the water. "If you keep fucking me like that, I guess not."
He can't fight the grin as Katsuki cackles.
"I guess I can do that."