It's a self esteem thing. When somebody thinks there's something physically wrong with them, it's stressful. It's not the kind of stress you deal with quickly (like, say, seeing a snake or losing your job), but it's the kind that stays with you forever, eating away at you, like an addiction or some tragedy that you can't deal with.
This kind of stress is dangerous, because it manifests itself differently in different people. In the case of somebody that's overweight, they likely deal with stress by eating. It's almost like an addiction, doing it makes them feel better even if they don't realize it.
A side effect of this (and probably the cause, depending on wording) is low self esteem, and self esteem that gets lower and lower. People like that, unless they wake up and realize what's going on, will eventually end up in an abusive relationship. Somebody who hates themselves will latch onto somebody who will tell them what they expect to hear, and tell them what to think and how to act, since they obviously can't do it themselves.
Like I said, it doesn't happen in every case, but it's a dangerous consequence of this kind of behaviour.
And by the way, this is coming from somebody (albeit a guy) who's overweight and has had issues with self-confidence. I've personally broken out of that kind of cycle, mostly on account of the kind of person I am, but I totally understand what they're going through.
