Anti-psychotics can affect some people's brain neurotransmitters and fat storage mechanism causing weight gain that is not related to diet and/or exercise. For most people the weight gain is *medically insignificant* meaning it doesn't affect their health but for some the pounds just pile on. Restricting diet and exercise can reduce as much weight gain as it would on any diet but it won't stop these metabolic pounds.
Personally, I've never gained more than 5~10 pounds on any of them and the weight fell off within weeks of stopping the drug.