The frost line is the depth to which soil freezes. Middle Earth Dwarves may live below the frost line, but I’m sure no people anywhere in the real world do. I’m not at all sure what you meant to say here. Not snowline, perhaps below the extent of permaforst?
Anyway I can’t see it being true. There is a general principle that says that the warmer the climate the taller the people should be genetically because of heat dispersion, but genetics is only a small part of the picture. I can’t imagine any general principle which suggests that cold climates would result in increased height, and no such pattern seems obvious.
The tallest people in the world for millenia had traditionally been African herdsmen such as the Masia and Watusi, and even Homo erectus averaged a little under 6’ in tropical Africa compared to 5’2 in colder areas. The Hottentot and Khoi people of the coldest southern parts of the continent were so short they were commonly described as pygmies.
Amongst Polynesians size shrank dramatically as climate became colder. The people of the South Island of New Zealand were some 6" shorter on average than the people of Tonga and notably shorter than the people of the northern most parts of the North Island. No Polynesian territory was permafrost.
In Australia the tallest people were to be found in the most northerly parts of the continent. None of Australia is permafrost.
I’d have to see a reference before I believed this for a moment. The tallest Asians are normally held to be people from Central Asia and Russia. I have never heard of Russians or Iranians considering Siberians to be particularly tall.
Not sure about North America, but the people of the extreme southern part of South America have always been descibed as short, stunted or underdeveloped. There is one group from Tierra del fuego, the Onas, who were described as being monstrously tall by the earliest writers, something like 8’ for the men. That story is supposedly where the name Patagonia comes from, being a reference to a legendary Spanish giant. But either they shrank dramatically following European contact or this was just a “here be dragons” story. Moreover the Onas lived well north of the much more frequently contacted and well documented shorter people, certainly not in the extreme south.
The tallest Europeans today may be, but that wasn’t the case even 50 years ago, when the tallest Europeans were Australians, New Zealanders, South Africans and Americans, who were largely of English and Irish descent. IOW those people who came from mild European climates and moved to tropical climates almost invariably outgrew their parents and all other stay-at-home Europeans.
Which simply shows that diet is the main determinant of size, not physical environment. In some parts of the world colder climates will produce better diet because of the effects of past glaciation on soil. But there are many parts of the world where exactly the opposite is true.
Just to complicate things further height will vary markedly within groups. The Onas may have been a respectable height, though not 8’ giants, but they were indisputably neighbours to people who were no taller than 5’5" Europeans. Similarly some island groups around New Zealand were descibed as of normal height while their mainland relatives were short. This sort of variation makes it very hard to generalise anything on a continental level.