weight gain after being spayed

i have an older cat, pixie, who’s obese. when she was about 4 months old we got her spayed and her brother, jack, nurtured at the same place and same day. jack maintained a healthy weight while pixie started to gain weight. we don’t over feed them, we play with them often but still she is obese. does anyone else have this problem? did something happen in her spayed surgery that would have caused her to gain weight and not be able to lose it?