The thing is that a butt is a muscle humans have a pretty good radar when it comes to telling how "in shape" that muscle is overall. It's just like you probably like big, powerful looking muscles on guys but that doesn't mean you don't like smaller, tighter, and more defined muscles (this can be anywhere, arms, chest, butt, legs).
The butt is just like that. We dudes like all types of butts, big ones, small ones, tight ones, whatever. There's a million different types of people out there so there's a million different butts we can like.
The reason why you always hear "big round butts" being talked about is because before you get naked, all we can see is the size of your butt underneath your jeans (I mean unless you're wearing revealing or tight clothes). Fitness, tightness, the way it all "hangs" and sits in position, and amt-of-cellulite don't matter at all, only size and general shape. On a side note, this goes for boobs as well, big ones are the only thing we can really notice through clothes, but when the shirt comes off, suddenly there are some smaller ones that become the most amazing things we've ever seen.
And I know I named a bunch of categories we judge on but don't get hung up about if you see a tiny spot of cellulite when you pinch your butt fat or whatever. I mean if we can see you're in shape, it's a good butt, there's a whole lot of fudge room honestly, you just gotta be in shape and not look out of shape. And if yours is how you describe it and you're not beefing it up (no pun intended but lol) then your butt is hot.