A snappy comeback that I have been dying to use should the opportunity present itself and somebody calls me "fat":
That may so, but I can go on a diet. Whereas you are stuck with that face the rest of your life.
I came up with this after being teased mercilessly about my weight in my youth. Sadly, I didn't come up with this response until I was older
(Is it still a "snappy" comeback if you have had it floating around in your head for a long time?)