What is the difference between a cow and human skeleton?

For example, pelvises of cats are much narrower than the humans. why? because humans have to walk, with two legs. but cats don't. they have their weight balaced into four parts.so, their pelvis' don't have to be wide. but, humans, walk with two legs, which will divide their weight to only two parts, their pelvis, has to be wide, to balace out the weight. for another eg. frog's thighs are much longer and thicker. why? also, frog has to jump and leap to other places. which will advance their thighs to be long and thick.