[sudosysgen]

Your weight is an equilibrium between calorie intake and caloric expenditure. Increased weight increases baseline expenditure, hence the equilibrium.

If you stop exercising, but keep the same caloric intake as before, yes, it will rebound really fast, because the equilibrium point will stay the same. If you want to move the equilibrium, you either need to do the exercise regularly forever, or reduce your caloric intake, or build significant amounts of muscle (fat free body mass) to increase your baseline. expenditure