[jules]

If you sport 6 hours a day and do not eat more you are going to lose weight extremely rapidly since you will need at least 4000 kcal just for that sport (compare with a normal daily expenditure of 2500 kcal). If your body really does use significantly fewer calories for the same activity as a normal person I would be very surprised. Millions of years of evolution have already optimized our energy efficiency to the limit. You probably wouldn't survive on 700 calories per day even if you stayed in bed all day, let alone living a normal lifestyle. In fact, just maintaining body fat requires calories, so if activities are otherwise the same an obese person burns through more calories.

That isn't to say that there are no genetic differences that influence body weight, there certainly are! It's just that the mechanism isn't energy efficiency, it's genetic factors that influence how much a person eats and how active a person is.