Cactus seems to have a lot of the answer as far as I am aware. Rear end/hips appeal to us visually because we're subconsciously calculating her ability to make an easy delivery, her health and fertility. That doesn't mean much, but it is what it is. We're always subconsciously scrutinizing ourselves and others according to standards that aren't readily apparent to us. It's biologically hardwired. 

I'm gonna get a groan, but checking out a book Scribz recommended to me once called "Sperm Wars" by a biologist named Robin Baker, is, at the very least, an interesting read for the layperson, though I'm not sure I'd call it a primer on the subject for non-scientific types. I leave it to the scientific types to give a qualified opinion.