I hit this problem when trying to backup some data sets. There was plenty of free space on some of the volumes, but not much free space on the “user” volumes.
You can use ISPF 3;4 to list data sets on a volume or with a specified high level qualifier. You can use the sort command, for example sort tracks. This reports on number of tracks allocated sorted in descending size. You can deletes unwanted data sets.
If you go to ISPF 3;4 and specify option V for VTOC, and specify a Volume Serial, you can display information about the volume.
If you are running in Unix Services
du -a ./ | sort -n -r | head -n 30
Gives the top 30 files or directories