B.S. back to you. Stomach size isn’t relevant unless it has been artificially changed. People vary in what they can burn. Some enviable people (possibly, depending on what climate they live in) even have “brown fat” which burns energy just sitting there. Others can walk all day and burn the same average calories as the normal person who is a couch potato.
If the kid stopped eating Big Macs and ate celery and salad for 4 weeks, he’d drop an easy 50 lbs.