Just an observation and maybe a lore question. It seems (only based on skyrim gaming experience) that the frostbite population seems to have recently started to grow based on quests and observatiobs. They have taken over mines and ruins and even the wilderness.

Is it tied to recent falmer activity? They seem to be friendly with falmer? Size due to magic? Dwemer?

Population same as other tes games?

Dragon age series had the same thing and believed it was due to dwarves

Frostbite spiders an Falmser are only in Skyrim, so there's no way to compare them to previous games. The Falmer are definitely a growing problem considering people didn't even believe they still existed an in the last era, but I don't think there's anything saying frostbite spiders are any worse than they ever were.