I am working on a setting with a large native population of humans who live in a year-round freezing environment, somewhat akin to the real world Antarctic. These humans should be as biologically similar to other populations as possible, however they will be taller and thinner on average than other humans, around 6'0" to 6'4".
It is important that this is a long-term population, not a group of recent migrants. Bergmann and Allen's rules suggest that this population should have adapted to be shorter, stockier, and fattier than the tall and lean populations of warmer, arid environments. What reasons can I give to explain this unusual set of adaptations for their climate?