That is legitimately a good question. I’m tempted to say the reason why it’s so well-toned is the same reason why machamp are, but machamp train in naturally rugged environments. We know so little about the Ultra Beasts’ home dimension that I can’t rightly say that buzzwole’s physique can be credited entirely to it without knowing for certain what that terrain is like.
Hmm. Maybe I should create a portal to the other dimension, just to see.
Bill, there is a really good reason why this is a bad idea. Her name is Lusamine, and her very nice and somehow not traumatized daughter is staying in your laboratory-slash-cottage right now. —LH
The Pokédex According to Bill 