Or this?
If that’s hand made, and it certainly may be, it makes my fingers hurt just looking at it.
I’m confused. First you say that down is low bulk, then you praise it for its high bulk. Just what does “maintain volume” mean, if not bulk?
In the winter, I wear a balaclava made from 100% synthetic materials. It keeps my ears nice and warm, while also being so thin that it doesn’t change the fit of my bicycle helmet. I also have wool ear warmers, that keep my ears just as warm, but there’s no way I could possibly wear my helmet over them, because they’re way too thick.
Its just knit. Two sticks and we’ve been doing it for centuries.
Bulk = weight and compressed volume. Maintaining volume = maintaining elasticity to expand in use to incorporate a large amount of trapped air.
For a given level of insulation (in dry conditions), down sleeping bags or clothing weigh the least, and compress down to the smallest volume when not in use.
I remember in the '80s that Thinsulate™ was all the rage, particularly in gloves.
I read that as well, somewhere. Amundsen and his expedition instead wore furs and leather clothing that they had got from the Inuit, reasoning that the Inuit were the experts in clothing for extreme cold conditions.
Yeah, it certainly looks like it would take that long to make. 
I could make that in less than six hours (those are huge stitches on big needles with thick fiber) and I’m not a fast knitter.
Wool is still more breathable and doesn’t get so stinky so fast as many modern synthetics; it also tends to degrade less quickly over time. But it’s more vulnerable to surface abrasion and slower to dry when wet.
But my sleeping bag eventually became thin and flat. I think this may be what Muffin meant:
If properly dried, it also releases heat when it gets wet: absorbing water is an exothermic action.
So true. My mother and sister had seal skin boots that they only used on the coldest nights because they were too warm in them most of the Southern Ontario winter.
In an adjacent thread, there was a good response from the point of view of an old-fashioned clothes wearer:
Their bottom line was that modern materials deal with being wet much much better, and can last longer (especially around hungry bugs/animals) but furs can be pretty warm and reasonably light.
I’d add that modern wicking underlayers are also a big improvement. Before 1970, a major skill of mountaineers was pacing oneself so that sweat never accumulated on ones clothes (which would make them either non-insulating, or very hard to dry). Nowadays, don’t need to worry about that nearly as much.
With thick wool clothing (think lumberjack) there are two problems.
The first problem is that it is too breathable – wind cuts through it, so it’s fine for calm days but not good on windy days.
The second problem is that it absorbs water which then freezes to ice which stops its breathability, resulting in your perspiration in your wicking and insulative layers then freezing up. When I come in after a night of skiing at between -20 to -30 c, my wool pant are frozen solid below my knees from the snow spray, which never happens to my poly insulating and exterior layers.
Much of the superior winter performance of Inuit clothing is from the use of windproof skins and relatively baggy designs that permit enough flow of air to vent excess humidity. There’s a brilliant movie of a 500 year old legend in which the cast wear traditional Inuit clothing that will give you a good handle on how it functions: Atanarjuat The Fast Runnner. Here’s a link to it: Atanarjuat The Fast Runner Zo Cd 1 : Free Download, Borrow, and Streaming : Internet Archive (Violence, nudity.)
On a trip to Iceland a little while back we were browsing in a shop and they had reindeer hide for sale. I went to put my hand on it expecting it to be coarse and springy and was astonished how deep the pile went and how soft it was. It was immediately clear that anything made from that fur was going to be incredibly good at trapping heat and keeping you warm. I never bought one but next time I will, just so I can have it on the floor and use it a foot-activated stress ball of sorts.