If you are using SQL database:
Migrations from a SQL database require 1GB available on the destination drive of the SEPM.
If you are using Embedded database:
The SEP 12.1 SEPM upgrade installer requires three times the current database size (sem5.db) if upgrading from pre-12.1 SEPM with the embedded database only.
I see that you are upgrading from 12.1.4 to 12.1.6. Hence there should be no problem with the disk space. However, to be on the safer side, keep free disk space that is qt least equalent to 3 times the size of the sem5.db file.
Reference: Planning for Migration to Symantec Endpoint Protection Manager 12.1
http://www.symantec.com/docs/TECH162730
After the upgrade: The technology of definition store by SEPM has been changed since 12.1.5 and hence after the upgrad the disk space occupied by the SEPM content store will be considerably reduced.