I haven’t read much about the heat fact but the less body mass you have, the more you’d have to burn for heat (there is a relationship between body size and metabolic rate which is inversely proportional, small mammals and infants burn insane amounts of energy just keeping warm).

I did read a study on non exercise thermogenesis and how some people managed to burn quite a bit of calories just fidgeting throughout the day.

So while we all run around the same human body temperature and thermodynamics is all the same, different people arrive at different results because of different behaviors. To what extent they are genetic is up to debate but much is genetic like “a tendency to fidget” or “a tendency to like sweets”.