Dropbox never deletes cache files
https://news.ycombinator.com/item?id=1752734(aka how to reclaim multiple gigs back from your runaway Dropbox client)
The documentation says it deletes after a 3 day cycle but this is completely untrue. I still see cache directories from several months ago.
If you look at your cache folder (~/.dropbox/cache for Mac and Linux), you might be very surprised at how much space is used. The current solution is to stop Dropbox, clear out the cache folder and restart which inane at best.