Some psychiatric medications can cause weight gain. Some can cause metabolic changes that lead to weight gain. Each medication has a different side effect profile and with some weight gain is less of a concern but each person responds differently to each medication. This is a list that could be used for informational purposes to discuss with your psychiatrist:
Weight reduction can be helped by an appropriate diet. You could discuss that with your doctor. If there are further concerns they could refer you to a nutritionist. If the current medication you are on is causing weight gain you could discuss this with your psychiatrist and they could adjust and/or change your medication as appropriate.
I gained weight from my mood stablizers as well. Nearly every person I meet who take psych meds has a belly that sticks out. Sometimes small and sometimes big. It's all from the medicine.
Personally, I am just trying to accept my belly and eat as healthy as I can. We have better stuff to worry about anyway right?