I have 3 children (ages 9,6,3) and have learned a lot about natural thin-ness from observing them. My 9-yo is naturally heavy...has been since she was a baby. Starting with breastfeeding, she was the baby who NEVER turned down an offer to nurse. She always loved to nurse. and as she got older she never turned down food. She was a very placid baby and is a very placid child. She was happy as a baby to sit and watch the world go by. As an older child, by herself she loves to read, draw and sometimes slowly scooter around outside. She'll ride bikes or play sports with friends, but her general nature is to not hurry, to take things slowly. She's happy to spend long periods of time sitting or lying down and reading or drawing or just playing. As a 2-yo, she would happily sit on our laps for an entire 1 hour of Mass and then another hour of a bible/study afterwards. And, she is (and was as a baby) on the heavier side.
My 6 yo and 3 yo are naturally thin. And, I notice a HUGE difference in their temperments. As babies, they were more active and crawled/walked/ just moved more. The only wanted to nurse when they were hungry and refused if they weren't. They are both fussy eaters...they aren't picky necessarily in that they eat a wide variety, but they are fussy. They don't like to eat that much, tend to eat small amounts. I can tell, they don't like the feeling of being overly ful, so they stop eating before then. They have days where they seem to live on air and not much else. And, they are more naturally active. By themselves, on their own, they spend more time running/jumping/moving They don't do well with sitting down for long periods of time and neither one of them could even really sit through 1-hour of Mass as a 2-yo.
Anyway, I guess what I have observed is that my thin children naturally are more active, they just naturally move more and they just seem to "like" eating less. They like to eat when they are hungry, but they won't eat if they aren't hungry and they seem much more sensitive to feelings of fullness then my heavier child is.
I know they are just kids, but I definitely see a difference. And, it seems to go along with what I have observed with adults.
I've found that naturally thin adults tend to just naturally eat less, they think about food less, they are more sensitive to feelings of fullness and they tend to naturally move more. I don't mean they necessarily work-out more..just that they move more. They spend more time standing up, they are more 'restless", they "fidget" more, they "pace" more, etc.