Those that live in glass houses shouldn't throw stones. No one is perfect, and as such to throw a pejorative out there like "fat" lowers the discussion to a level that is reserved for the absolute lowest form of discussion. There are lots of things we all wish people didn't know about us, so why throw out insults like that?
If you do, the person you insulted has every right to call you something back, and there shouldn't be any retaliation.