I don't think that's how it works. People you perceive as "naturally thin" probably simply:
* are used to not snacking between meals
* eat when they are hungry, and stop when they are not
* do not drink (excessively)
* are more-than-average active
Calories add up extremely quickly and it's very easy not to notice if you don't pay attention. Say you have one Mars bar per day, as a snack - that makes a difference of 10 kg of body weight per year as compared to someone who skips such a snack (a back-of-the-envelope calculation).
* are used to not snacking between meals
* eat when they are hungry, and stop when they are not
* do not drink (excessively)
* are more-than-average active
Calories add up extremely quickly and it's very easy not to notice if you don't pay attention. Say you have one Mars bar per day, as a snack - that makes a difference of 10 kg of body weight per year as compared to someone who skips such a snack (a back-of-the-envelope calculation).