Yep, haven't drawn butts for a looong time. And this time I got a bit educational. And it gets more interesting when we're talking about big assets, of course, heh.
I'm starting to go to a gym, so I'm trying to remember all muscle names and being more aware of them (some years have passed since I knew them all from school, heh), especially in shape and movement. Learning basic anatomy to draw in a realistic way is a must, and as I don't practice this too often, now I feel kinda motivated.
Note that I'm not an anatomy expert, I know this just by plain observation (and experience).
Btw: Any preference? A well shaped muscular butt: nice. But, as some ancient and wise poet said once: "I like big (fat round) butts and I can not lie".