It’s called body dysmorphic disorder.  It is a general problem where the user does not see themselves as the rest of the world sees them, and they come to focus on one or more problems that they seem to see with their body.  There are some who go in for regular surgeries, and it could be said that Michael Jackson suffered from this particular problem.  But most often, it seems that people who suffer from this particular disorder also suffer from anorexia nervosa or bulimia nervosa.  They therefore try to change their body in extreme ways, always seeing a sort of distorted image that makes them think that they are overweight, even comparing themselves to other people who are actually bigger than them and talking about how they wished they were that skinny.  It doesn’t make any sense, and likewise it leads to extreme behavior.  However, there are of course various ways that individuals manifest this particular problem for themselves.  It can lead to bigger problems both mentally and physically speaking, as of course anorexia and bulimia do, and of course if they go in for surgeries, the surgeries can go seriously wrong among other things.