What it means is that to be 5lbs underweight is exactly that: below your healthy BMI range. In that case, you'd be rapidly approaching anorexia-land, or like you said: have some underlying condition causing said weight-loss. They're comparing being 5lbs closer to emaciation with the infinite mass one can gain before exploding. It's insane. Sure, 75lbs per -5lbs, let's say. That underweight person will be dead before you get to 500lbs— but we all know the fatty won't stop there. *edit: I'm pointing out how even if their delusion were real— they still can't help misinterpret the information to justify their stupidity.