I have asthma, which makes me more susceptible to bronchitis and pneumonia. Therefore, I am usually on prednisone at least once or twice a year. I'm on it right now as a matter of fact.
It almost always dramatically increases my appetite, and also water retention/bloating/puffiness. If I make reasonable food choices when the insatiable hunger kicks in, then I may only gain 1#-3#, and that weight has typically dropped off within a couple week after stopping the steroids.
Good luck. BTW, I hate being on prednisone, but sometimes I have to put up with all the yucky side effects in order to breathe. It sucks.
Hang in there.