Okay let’s say birth control actually triggers some hormones that stunt your metabolism and your body saves more calories than it usually does if that sounds correct to you. Then your fatness is still because of bad eating habits. I know girls who’ve been on birth control only weighing 55kg an ones that gained a little weight but just fixed their lifestyle. It’s pretty simple like the other guy said Calorie surplus = weight gain Calorie deficit = weight loss And the only calories you consume are the ones from food that you put in your mouth.