Could be because every time IR is started, it adds a new image to the PV history, filling up the PV much faster than you might normally fill it up. You could find where C4D is storing the Cache files, and check how large they are. And you could try clearing out the PV history regularly to see if that is where the majority of the cache is coming from. I was always surprised to see just how much history the PV was storing :) There are probably settings for that too, to control how much history it will store.