Caused by the fat person's feelings that society looks down on them for being fat. Usually more common in females since they are typically more ostracized for being overweight.
Victims become obsessed with losing weight or (unfortunately, more commonly) just trying to make themselves appear less fat.
Diet pills, miracle weight loss programs, etc etc.
Talking about exersize/eating well, caring WAY too much about dressing nicely (even in casual situations).
Some try to be overly sexual, ie hugs to EVERYONE, tattoos (esp. tramp stamps), dressing like sluts, being strippers, talking about sexual things, overall trying to make themselves more sexy
Unfortunately, attempts are usually counterproductive. Trying to act slim only pisses people off and makes them notice obesity more.
Closely related to short people syndrome