How can I avoid weight gain on birth control?

I'm going to be starting to take birth control pills soon...I've heard yaz is really good. My main thing is that I don't want to gain weight. I could understand water weight, but I have a good diet (low fat vegetarian, no junk food or refined carbs) and I don't want to get on a pill that will make me have pregnancy-like cravings. I'm sure my doctor can help me with this, but I prefer hearing from girls/women who have had personal experience. Thanks!