I noticed the other day that the derived data cache was filling up a large majority of my drive space (>60 gigs) when the editor crashed from lack of disk space, causing a loss of work 
It seems that when editing geometry brushes on a map with many (>100) of them, the derived data cache fills up rather rapidly.
To test this, you just need a map with a large number of brushes (here’s one of mine: Dropbox - cq_excavation.umap - Simplify your life). First observe the size of the derived data cache. Then in the map, make 20-30 edits (adding/removing/resizing/moving brushes) and observe the size of the derived data cache again.
On my computer, 20-30 edits fills up a good 2-3 gigs of space, which is a lot considering deleting the cache and then opening every map in our project fills up ~10 gigs.
My temporary solution is to save a copy of the cache with only the necessary stuff and then replace it when needed.
Judging by the number of files with “PHYSX” in them, I’d speculate that the collision generated when brushes are built isn’t getting cleaned from the derived data cache? Just speculation of course.