I think a lot of their muscle size and thickness comes from time under tension. Although I'm not much for worrying about TUT, studies show that if you keep heavy, continuous tension on a muscle for more than 60 seconds you can stimulate serious growth.
As far as bodyweight movements not being able to develop an awesome body, just go to youtube and search for "hannibal for king". He's not too big, but his strength to bodyweight ratio is freaking amazing, and he's shredded.
Another big thing is adaptation. Most new trainees can't train much more than an hour a day without wiping themselves out both physically and hormonally. But if you do it almost every day from the time you're very young, your body gets used to the volume and you can use it to train crazy hard.
I know, these are just a small part of what's going on, but I think it's still a part of it.
Current PRs at 242- 495, 345, 585, 1425