Two things come to mind:
-
If your instance was populated with a restore, or imported from another platform you may be carrying the restore schema around:
https://meta.discourse.org/t/disk-space-usage-by-postgres/61137
-
backups left on disk have filled your space:
- Check /var/discourse/shared/standalone/backups/default
- The backups are .tar.gz files
- Look at the date AND size of the backups before you start deleting.