I always hated the word "fat". I prefer big. Fat sounds derogatory and I prefer to hear someone say I have a "big ass" as oppossed to a "fat ass"
If you showed me a photo of a "large" ass and using the same photo but called one big and one fat I would always want to pound the big ass. I have been with quite a few Rubenesque ladies and I can only tell you that there is nothing in the world like pounding a big ass from behind (pun intended)