How is my friend not 500 lbs?

ok my friend is a 25 year old guy. He is VERY sedentary to say the least. He eats huge meals like giant bowls of cererial. He will usually eat a good sized lunch and go back for 2nds and 3rds on dinner. He also snacks a lot. I see him knock out prob 700 calories just munching on chips and thats just a fraction of the snacking. He drinks soft drinks like its water. Having said all this I will now say that he is 150 lbs and 5'11". His brothers are like this too. How the hell is this possible because if I did this for a year I would be 800 lbs.