How do some fighters get so big without heaps of weightlifting?

It’s difficult enough to get big when hitting the gym, but why are guys like Wladimir Klitschko who i’ve never seen lifting heavy in the gym so big?

I can’t see this resulting from just bodyweight exercises and boxing alone?

And even if they lift they can’t afford to spend a whole lot of time doing that either.

What’s up with this?