I don't understand this, if we have a society that hates on fat itself, then why the fuck do women like these want a bigger, "fat" ass?
Because we have a very diverse society. The definition of beauty varies depending on region and even depending on the individual. Certainly if you only look at fashion models you might think that society hates fat... Except that even in fashion world you will find some XL models, not as common as the skinny ones but despite how anorexic the average model and how that is the image the fashion world has wanted to push, they have begrudgingly begun to admit that there is a demand for clothing for overweight people as well. (Certainly overweight customers are more common than the anorexic ones.)
And I only made that example because the fashion world is the part of society that has in the last decades been the biggest pusher for "only very skinny people are pretty" idea.
In fact, if we look at a bigger picture, saying that the society hates fat is a huge oversimplification. We have subcultures and fetishes that admire obesity. (Ok, that was on the extreme on the other side, but it does exist.) We have the mainstream people who might not consider extremely obese as their image of beauty but do not go for the other end of the scale either. Our society(s) is(are) really not that homogenous.