The actual location of the Client Packages you referenced is:
C:\Program Files (x86)\Symantec\Symantec Endpoint Protection Manager\Inetpub\ClientPackages
If you deleted unwanted client packages with Admin>Install Packages>Client Install Package>Delete Client Install Package, the size and contents of this folder will be reduced
"ImportPackage" being large is a completely different issue
Stop the SEPM service and Symantec Embedded Database service and delete the contents of:
C:\Program Files (x86)\Symantec\Symantec Endpoint Protection Manager\data\outbox\ImportPackage
Restart services or reboot the server
You may want to upgrade to SEPM 12.1.4; you appear to have added the new Mac package to your RU3 SEPM which is supported but best done by the SEPM upgrade to get the new SEPM features that go along with the new Mac client.
MJD