Overeating is not a mental disorder, it is either a choice not to concentrate on the act of eating food and on the concept of feeling full, or it is a choice to keep eating even though you feel full but are too in love with the food to stop. It can also be a medical disorder where a person does not have the sensation of being full so they do not know when to stop or understand the concept of stopping even though they do not feel full.
BEING overweight can also BE a medical disorder and not necessarily connected to the amount of food a person eats.
This is in contrast to eating disorders which are an issue of being in control of that part of your life and your body. Eating disorders are very scary and dangerous because it can and does lead to death.