There is a known issue with Symantec Endpoint Protection 12.1 clients where it does not delete the old virus definition files. These definition files build up and start consuming gigs and gigs of disk space.
They do have a fix for it now, which involves upgrading to Update Rollup 2 (UR2). Unfortunately, you might not be able to upgrade right away. As a workaround, I wrote the powershell script below that will cleanup the old definition files. I schedule this to run daily.