If you want to ruin their day, tell them fat people have a higher metabolism because it takes way more calories just to maintain all their extra body fat.  It's true.  Just look at a BMR calculator.  The more you weigh, the higher your basal metabolic rate is.  In simple terms, doing nothing, they burn more calories per day than you.  They're just replacing all those calories by eating too much food.  If they simply ate less, they'd lose weight. Science, bitch.