The CQPOweb database that is supposed ot keep track of the cache contents is losing track of some files.
In my cache directory:
select sum(file_size) from saved_queries
Default cache limit: (which is NOT overridden): 3221225472.
I suspect that crashed processes result in files on disk without matching entries in the database. The files are, thus, lost track of.
Solution: admin interface for managing "orphaned" files - if possible, also add an option for superusers to be emailed with a warning if the orphaned files overspill.