There's a subtle something about how Ichigo mocks Rukia's size that's different than when he's mocking anything or anyone else.
It's written faintly in the slight strain that appears on his face and in his voice, and if you watch carefully, that same strain releases whenever Rukia inevitably pushes him back with vehement protests, making everyone grin. Watch for long enough, hard enough, and it becomes a heartbeat—strain, release, stain, release, ba-thump, ba-thump, ba-thump.
It's a subtle heartbeat, and it doesn't nearly beat as fast as Ichigo's does whenever Rukia's there, but it certainly beats as strongly. Nobody really knows why it's there. You'd think Rukia would be able to tell, given her expertise with Ichigo's heart, but Rukia turns out to be pretty stupid regarding some matters of the heart. Why she's that stupid is a pretty long story, but suffice to say that the part of the heart that Ichigo can't help but feel from when she's around—ba-thump, ba-thump—well, her part got hurt a while back, and Rukia refuses to let the scar scab over. Penitence, or some stupid shit like that.
But Ichigo, for once, has a pretty good idea of why it strains and claws at him inside to mock Rukia's size and why he does it anyways.
It's a pretty deep-rooted fear. Ichigo thinks it started way back with Chad and the parakeet. That had been a disaster. He'd had to run home and drop Karin off, and by the time he got back, Rukia had really pulled a number on herself. He remembers a torn school uniform, what seemed like too much blood for one body to spill, skin that was pale even for her, and lavender eyes that wouldn't despair. He remembers thinking that she was an idiot, feeling a grudging admiration, and then further beneath that, an all-encompassing fear.
What if he lost her?
Several months later, with his hollow finally beaten down, he finally knew the answer to that question.
If he lost her, he'd lose himself.
God, Ichigo didn't have enough conviction, enough heart, to do the things that one whose name was protector should do. He'd been taking, he'd always been taking heart from Rukia, whose heart knew no bounds. He didn't know when, but at some point the raven-headed shinigami had served as his heart for so long that he didn't know where his own was anymore. She'd probably stolen it. And that, surprisingly, was OK with Ichigo. His own heart had been weak anyways. He'd lost so much of it when his hollow reared its ugly masked head.
Well, it was OK with Ichigo most of the time. When your heart is permanently within the hold of a headstrong heartstrong idiot of a shinigami, you worry about your heart a lot. So he's overprotective. Wouldn't you be if your heart got into as much trouble as Rukia did, and was trapped in a container as frail and tiny as Rukia was?
But Ichigo can't say any of that. She wouldn't believe him, she'd call him a pansy, she'd be a complete idiot about it, he just knows it. So he teases Rukia about her size, and lets himself be reassured by her responses, lets himself laugh.
He does it because the alternative is to smother her in his grasp and cry.
