it's genetic plus life habits

but mostly genetics. I never eat lunch, just two meals a day, big dinner usually, snacks between meals. Very lean and healthy, 19% body mass, perfect for a young woman. Less than 19% fat prohibits bearing children. When I was an adamant runner, I have only 15% body fat. My dad is very thin too. My dad's mom was skinny. But my mother's side is plum size. I didn't inherit their genes evenly I guess.