I don't know whether this is physics or biology. I think that depends on the answer as much as the question. 1) Hollywood says a big hulking giant would be a huge threat. 2) Critics laugh and say that a big hulking giant wouldn't even be able to walk. 3) Horses are fast, mice are slow, and ants are just pitiful. Sure, ants are pretty macho in proportion to their mass, but they can't run that fast. So why? Is there a point where this trend is reversed? Elephants are pretty fast, too.