I think it all began with the sub-conscious beliefs rooted with picking a woman who's perceived as better child bearers . I think the music industry also fuels the notion as well with lyrics like "I like big butts and I cannot lie", "babies got a booty like pow, pow, pow". I think it's appealing for guys to have some something to hang on too and visually appealing during sex. A slim woman can also work on getting a rounded butt by doing lunges and other butt enhancing exercises.