How to limit cache folder size My runtime/cache folder is over 200GB in size. Is there a config setting or some way of limiting the size of the folder, rather than the cache duration?

Secondly, what could cause such disk usage?
There's not a native Craft way to do that.

You could setup a cron job that hit a plugin's controller and delete items in cache based on size/date created, etc.

My guess is you've got a plugin installed doing some heavy caching. You can try opening the largest cache files (it's all just serialized text data) and see if the type of data gives you a clue to where it's coming from.