User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4212769-20120309101156/@comment-1011548-20120310093644

Lots of the Tundra habitat consists of caves. Perhaps monsters that grow too large find some of the smaller caves too uncomfortable to navigate? Beyond that I'm not really sure. You can get small things in cold places, though; lemmings and the weasels that hunt them come to mind.