[Dewie3]

The ratio of "livable land" is also lower. Crops and humans seem to have that thing in common, not wanting to live at high altitudes or at the bottom of fjords. Though of course livestock don't mind grazing at higher altitudes, in the summer. And maybe having to have smaller farms leads to less efficiency of each "unit" of farmable land?

On the other hand, Japan is pretty mountainous and still has a high population density.

Norway probably didn't have that high population density 1000 years ago, either. But it was enough to cause overpopulation, and in turn motivate the events and practices of the Viking Age.