If not sexual, some other type of abuse of mental trauma. I have believed for a long time that for most people obesity is just a side effect of mental health issue.
Being VERY fat or being very thin both take much effort. The amount you have to eat to be 600lbs+ is not an enjoyable amount of food. Eating is a compulsion and form of punishment at that point.
You could say that someone that fat is disciplined--takes more effort to be that fat than to just be overweight.