By default...you CAN'T be unhappy in heaven and being fat would make me VERY unhappy! LOL!
Well, first of all, the definition of fat is relative. Some think a size 10 is really fat, some people think size 10 is normal, some think it's too skinny.
Think about it, during the middle ages, fatter people were considered more attractive than thin people. Tastes change over the millennia.
Still the idea of being able to eat all the carbs you want and not gaining weight would certainly be heaven to me. LOL!