Here's some advice from my experiences working in a reproductive clinic:
The birth control pills themselves do not cause weight gain. Changing your eating habits/lifestyle does cause it. Being on the birth control pills may have curved your appetite or caused you to crave foods that led to the weight gain, but the birth control pills themselves did not. That was an old myth and is no longer listed as a side effect of birth control.
The only exception to the weight gain issue is the Depo Provera shots. They do not cause you to gain weight, but they can make it very difficult to lose it.
I personally was on depo for three years and put on weight after having my gall bladder removed. I had a hard time losing the weight, until I went off depo. I switched to Ortho Tri Cyclen and the pounds began melting off my body.