Humans have such big butts( especially here in the U.S) mainly because of poor eating habits, and poor lifestyle habits. Simply put, the body is meant to move, not sit still. On the other hand, studies show that women with wider hips can carry a child and give birth easier. That's why most men are naturally drawn to curvy women.
I always thought it had a lot to do with the fact that many human beings were naturally nomadic and many lived in hotter climates. Their bodies were naturally lean because they were so active and also to help dissipate heat. but they also needed fat stores and I kind of thought of it like the camel Theory. The camel stores it's fat in it's hump and I figured that human beings are naturally meant to store their fat in that area.
Similar to the fact that women like strong men, we look at curvy women. In my view, mother nature relates beauty to our capacity to survive. Strong men are seen by women as good providers and defendants of their brood, while women with wide hips, breast and legs, are inherently seen by men as good child bearers and mothers. Only an opinion