One of the common misconceptions that most women have is birth control pills causing weight gain.
Studies have shown that there are minimal effect and association between birth control pill and weight gain—if it even exists at all.
The oestrogen in birth control pills does affect fat (adipose) cells, making them larger but not more.
Women might be retaining more fluid, making them feel as though they've gained weight, particularly in the breasts, hips and thighs.

Reference: http://www.mayoclinic.org/healthy-lifestyle/birth-control/in-depth/birth-control-pill/art-20045136?pg=2/