Taking a look at where my disk space went, I was quite surprised to find 11GiB in a directory at ~/.wine/drive_c/windows/profiles/username/Local Settings/Temporary Internet Files/Content.IE5/
It appears that this is an already reported issue with wine, as downloads made through calls to wininet.dll are supposed to be deleted by Windows sooner or later. As wine caches its downloads the same way but without ever removing old files, everything ever downloaded through wine’s wininet.dll currently gets cached indefinitely. To work around that problem, it’s currently necessary to clear that folder once in a while (or to automate that on reboots or similar). Simply deleting that directory should work just fine as it is supposed to be created automatically if needed.
It’s probably best to also search for other temp directories inside ~/.wine from time to time. Apart from cached downloads I could free another 6GiB of unnecessarily wasted space at the usual locations. It’s easy to forget about “C:” when running wine… :)