It could be all sorts of things. Some people's bodies just aren't as resilient, whether its weaker bone density, smaller/weaker joints, ligaments, etc. You'll see some athletes who are extremely susceptible to minor things like muscle strains, whereas others aren't. Buxton might just be a guy who is extremely explosive/athletic but in a body that is a bit fragile. I don't think that his weight has anything to do with it, and really, Mike Trout's more robust frame just means that he's putting even MORE stress on his joints and tendons and ligaments any time he exerts himself.