I've seen this discussed on Fetlife quite a bit, and some of the opinions go like this:
Thin men prefer thicker women and vice versa (haven't heard from too many gays or lesbians on the matter) because 2 sets of hipbones striking together is actually painful and can leave nasty bruises.
Younger men prefer thinner women because they have been more trained by the media ideal, than by experience, to know that larger women can be extremely sexy and desirable. As they mature, their tastes change.
Kinky people are less concerned about body type than mainstream people because they look for partners whose kinks line up with theirs, and put less value into particular body type or size.
Oddly I am on the more voluptuous side and both my long term sweeties are thin to average sized.