This i something you needed to hav discused with your prescribing doctor, and I hope it was a psychiatrist in the first place, but maybe its one of those concerns you thought of on the way home !  This is why I always recommend that before going to see a doc, you think a while and jot down the questions you'd like to get answered. 
Many antidepressants can cause weight gain in SOME of the people who take them, but not in everyone.  So its far from inevitable that you will gain weight on whatever is prescribed for you, especially if you maintain a healthy diet and exercise balance.  If weight gain nonetheless occurs,  discuss this with your shrink, and alternatives can be tried.  Bupropion is the AD least likely to cause weight gain, and there can be a degree of weight loss.