From what I know, weight gain wasn't recorded in the clinical trials of Advair, but it's been reported since, so it's listed as a "possible" side effect, but there's no info on how common or severe it may or may not be.

From what I've read here (I'm not a medical professional, though), they recommend that if you are experiencing weight gain with Advair, you should report it to your doctor and see if your doctor has any suggestions or if there's an underlying reason for the weight gain not related to the Advair.

From my own experience, if I'm flaring badly, my appetite disappears, so I lose weight. When I get back under control, my appetite returns with a vengance, and I gain back all the weight I lost (since I'm normally at a healthy weight, and I tend to become underweight if I'm badly uncontrolled for an extended period of time, I don't mind this at all - I really am too thin when I hit 110lbs or less). It might be that your appetite is returning with improved asthma control. Just a thought.

If you're worried about your weight gain, you should talk to your doctor, and maybe to a nutritionist to ensure that you have a healthy, balanced diet (which is best for reaching and maintaining a healthy body weight).