I guess every person kind of has their "thing" that they're obsessed with when it comes down to kinks, this would be mine. Butts are a lot of fun for me because of how it can feel like such an intimate or flirty thing to focus on, without being an inherently "sexual" thing unless everyone involved is into it. In that way, having a character with an especially big butt, or one that's made bigger through other means [the author nodding sagaciously here], is an easy way to draw a center of attention on a character that wants a lot of it.

Of course, you could say something similar about a lot of other parts of the body, but I especially like a lot of things about butts. Hugging around, being underneath or between, or even -in- (yeah) is all great stuff about big buns. Though I think it's best when it really gets to the characters involved that it's something special, or to be shown off. Really, anything that feels 'gratifying' to how the butt matters is a lot of fun to me.