Blow off the question casually, but do continue dating her. Make sure there's plenty of food options at every date. Insist that she polished off every plate. When she's over at your place, let her relax and cook up a feast. Really encourage her to go all out, and that she can have a cheat day because you worked so hard to make all that food.
If you make it to the bedroom, bring up some innocent foodplay. Chocolate syrup, whipped cream, you know the drill. Be sure to do this after large meals. She'll begin to associate food with sex. Priming her to feel good whenever she eats, making it far more pleasurable to stuff herself.
After a while, she might start to grow a little distant, and that's probably because of the other ways that she's growing. She might be ashamed of the weight she's gaining. Ask her what's wrong over dinner, and get her to tell you. She feels that she's becoming unattractive due to her weight. Let her know that you think her belly is beautiful. Don't outright admit that this is your fetish- instead, act embarrassed and shy when you let her know that you have a thing for chubby girls. She'll comfort you, and bond over it.
Now you've got her where you want her. She knows that what she's doing may not be attractive to others, but your acceptance is what matters to her now. She'll voluntarily engorge herself, wanting to do her best for you, and play to your fantasies.
You've gotta be the one who takes control and give the girl the bodytype you desire.