Hello,
This is a normal. Eventually these files would be deleted.
I hope you have migrated to the Latest version of SEP 12.1 RU2
Symantec Endpoint Protection 12.1 will keep 1 definition set
Symantec Endpoint Protection 11.x will keep 3 definition sets at any time: the current set and the last 2.
Disk Space Management procedures for the Symantec Endpoint Protection Manager
http://www.symantec.com/docs/TECH96214
Drive Space used by Virus Definitions Updates
http://www.symantec.com/docs/TECH141811
Also, check this Thread with similar Issue -
https://www-secure.symantec.com/connect/forums/virusdefs-folder-over-10gb-size-servers
Hope that helps!!