Hi,
By default the GUP will automatically purge content from its cache under two conditions:
If the content on the GUP grows larger than the configured Maximum disk cache size for content updates setting. The GUP will purge the oldest content by last accessed time until there is room for any new content.
If any individual content is older than the Delete content updates if unused setting, the GUP will remove that content.
However with the release of SEP 12.1 RU5 SEPM uses content storage optimization feaure. With this method I believe client GUP machine doesn't need more disk space to download contents.
Content Storage Optimization feature:
As part of the upgrade to SEPM 12.1 RU5, the SEPM converts all of the content from full definitions to delta definitions. This process is resource intensive and may take an extended period of time. After this process is completed, the SEPM will use significantly less disk space.
In a typical enterprise setup where 30 content revisions stored, the SEPM upgrade process must reduce 55GB of full content to under 2GB of delta content. This process requires significant resources to complete and is impacted by the performance of any available CPUs, CPU cores (physical/logical/hyperthreading), memory, and disks (I/O). On a server that performs multiple roles, stores larger numbers of content, or is otherwise resource constrained, this process may take a longer duration to complete.
Refer this article to find more info: The LiveUpdate content optimization and content storage space optimization steps take a long time to complete when upgrading to Symantec Endpoint Protection Manager 12.1 RU5
http://www.symantec.com/docs/TECH224055