The weight gain associated with birth control pills is typically only 5lbs, at the most. I lost 60 lbs while on the pill and it didn't affect my weight loss.
However, once I went off of the pill I did drop a few lbs without even trying. That was only 2-3lbs though, so not very much.
I do believe that other chemical birth control methods, like depo and norplant can cause a larger gain, but that isn't necessarily true for everyone.