Dont even really have to be active, I recently started a new desk job that I need to drive an hour a day to and from. So ten hours sitting and then more when I get home cause Im tired from the early start. Havent gained any weight even though I havent done any sort of exercise, all cause I watch my food.  If someone were to actually ask me instead of saying "you're lucky you're naturally thin" I would tell them- eat exactly as I eat and you would be as thin as me