I agree with @Blackberry most of American society wants small and curvy. It does vary of course by ethnicity, culture, nationality, and individual. I think pretty much a very flat ass, not to be confused with a small butt, is not coveted. The pear shape, where the bottom is much larger than the top, is not thought of as ideal either. Balance and proportion are key in my opinion, it is the entire package the woman presents.
America has moved more and more towards a little bit of a sway back and bigger bottom, even manequins in stores are more shaped this way, especially in the Miami areawhere the Latin American popuation is strong. In my zumba class the instructor talks about having a zumba butt, not sure if that means firm? Or, bubble? Or, what? The emphasis on sway back really bothers me. Of course some women are naturally like that, due to their spine, but it seems people don’t know what good posture is anymore? I don’t get it. And, young girls walk around hunched over? Bothers me. Bothers me because I think no one is telling them to stand up straight. But I digress.