A while back I read an article on the reason fats tend to give up on diet/exercise so quickly. In short , it was basically saying that fats think its too hard to maintain and quit. We all knew this, but the author brought up the fact that you dont see fats out jogging bc of the intense physical strain, a walk for them is like a thin persons marathon. So exercise WHILE OBESE causes too much strain on the body and causes it to wear down quicker. Need  to find that article, it wasnt scholarly so it might be a little harder to look for...