As someone who takes ass very seriously, it's a pleasure to find a thread like this! For me, ass isn't just "nice" or "nasty". Ass is so much more, it's a science. As in, WHY is it nice? WHY is it nasty? Basically, there are five different types of butts on women, and I'll post an example of every one and explain.
1) The small, firm butt
This butt has less fat than the average female ass. That doesn't mean it lacks in shape though. It's a firm butt, because otherwise it wouldn't be shapely. But because of the lack of fat, it's small. Ayumi from X-Blades is a great example of this category:
2) The small, flabby butt
This is similar to category 1 ass, but with a difference, the flabby butt lacks the firmness and the muscles to hold it up and make it look perky. Still, it doesn't have a lot of fat, but what's there is just flab. Xianghua from Soul calibur II has this rare type of ass:
(large mp4)
3) The big, fat ass
This is typically the most liked butt, but also the most misunderstood one. The big fat butt is not a huge lump of lard. Rather it's actually quite perky and shapely, but there's so much fat on top of the glute muscles that it defines as a fat ass. This is what category 1 ass would be if you put more fat on it and therefore made it bigger. Vanessa Z. Schneider is a great example of a category 3 ass:
4) The big, fat, flabby ass
This is basically a category 3 ass, but without the perky shape. This is because a big, fat, flabby butt doesn't have enough muscle to keep a great shape. Instead, the shape is a bit saggy and droopy. Samus Aran from Metroid : Zero Mission is the perfect example of this rare type:
5) The big, muscular ass
This is very rare among women because they have less testosterone than men. That means that even if they do their squats, they won't build as much muscle as men do. This is also the reason why ass size on women depends on fat levels, why ass shape depends on firmness levels. The exception of that rule is category 5, which Chun Li is the greatest example of:
All butts fit into these five categories. If the woman's butt is big, it's most likely fat. If it doesn't have a round, perky shape, it's not firm. The key thing when it comes to firmness is to look at overall shape and the creasing lines between the buttocks and thighs - that's always a giveaway.
Great thread!