To my understanding, it is a combination of both. The larger the comb, the more vulnerable to frostbite it is. If you are worried that your chickens may be at risk for frostbite, put some vaseline on their combs.
Going by the experience I had with chickens some 25 years ago - they can take a lot of cold with out getting frostbite.
I set a section of an old storage shed aside for my first coop. The space had no covering on the windows, holes in the walls, and no doors on the building. I installed wire around the upper half of the space and covered the wire and window openings with 1.5 mil 'vapor barrier' plastic to slow most of the wind.
My chickens did very well in this drafty, ratty place down to -25 degrees. It was not until the temperature got down to nearly -30 degrees (F) that I had a few roos with frostbitten combs.
All that I have read here indicates that it is a hight humidity combined with the cold temperatures that causes frostbite. Since I know that first coop had NO problem with humidity, I'm gonna go with moisture rather than cold as a primary cause.