I don't know.
But according to Scrubs, the difference between a black girl and a white girl is that when a black girl asks you if her ass looks big in those pants, you better say "hell yeah!" Haha
If you wanna know why lots of guys like bigg butts, I suggest you listen to "Baby got Back" by Sir Mix-a-Lot. Lol.
But not all guys like big asses of course. Mine is petite, and at the time of the whole " J.Lo's big ass" phase the world went through, every women's mag had articles saying shit like "men don't like tiny butts... so shake your big booty!" and I got a bit insecure about my little ass. But since then I stared going out with my gorgeous boyfriend, and he loves it. I've always thought I was all wrong. But my boyfriend thinks I'm beautiful, so it's all ok!
Moral of the story: doesn't matter whether your ass is big or small, someone will love it!