by Ken Task.
Recyclebin? ... if a resource/activity (quiz) is deleted from a course, a backup of that resource/activity is made and stored in the sea of files moodledata/filedir/
https://docs.moodle.org/36/en/Recycle_bin
Those recyclebin files are retained for 4 days. Thus moodledata ballons a little.
Also in task there are specific ones for clean up of recyclebin.
\tool_recyclebin\task\cleanup_category_bin
\tool_recyclebin\task\cleanup_course_bin
which are part of cron but like other task are scheduled to run at certain times.
'spirit of sharing', Ken