from their website (norrona.com)(there you can also see the more specific sizing):
The human body spends a lot of energy keeping its optimal temperature, especially under cold conditions. Primaloft®One is a patented icro-structure that gives great thermal insulation to help the body maintain its temperature, minimizing energy loss. Extremely light and soft, it was originally developed by the US Army to replace natural down. As down absorbs moisture, it becomes wet and loses its thermal-insulating abilities. Primaloft® One absorbs three times less water, is 14% warmer when dry and 24% warmer when wet, than the competitive insulation.