Girls who get fat tend to get fatter, anon. You just have to coax it out of them.
Last date I went on was with this huge Mexican chick to a Cheesecake Factory. She was looking at the menu and said something along the lines of "oh, I don't want to order too much" and I responded "don't worry, my treat." That little cue opened the flood gates. She ate what must have been the equivalent of 5 normal sized meals in one sitting. When we went back to her place and fucked I just kept imagining all that food inside of her digesting and turning into more glorious fat.
Fat girls like food, if they didn't they wouldn't be fat. You just have to let them know that you're okay with their love of food, and they'll keep eating until they're too big to move. You don't even have to let them know you're turned on by it, just say you appreciate them however they look physically, and they'll stop caring about their appearance long enough to pop another slice of pie into their mouth.