I was with you until the last sentence. I'm from a whole family of skinny people. We eat like horses, just to maintain our weight. Nobody gets even a little tummy bulge until they're 45 or 50. My middle son is currently 6'3" and 150 lbs and eats constantly, the highest-calorie foods available to him, as much as several other people. Always has. When in high school for an after-school snack we baked one frozen pizza for the family, one for him.

This is clearly some genetic thing - I was the same way; my nieces are the same way (one getting married in a size 2, that had to be taken in). Eats ice cream by the quart, as often as she can get it.