Hi,
Normally a client should be pulling a delta file from SEPM for content updates. The average size of the AV delta file is around 200KBs to 300KBs. The client could be updating this 2 to 3 times daily. So it could range between anywhere between 400KBs to 900KBs daily for client's for content updates.
Our Internet LiveUpdate (LU) server will distribute delta updates. LiveUpdate Administrator (LUA) which can deployed inside the customers network will download files from LU to distribute to other Symantec products (such as SEP or SEPM). Since LUA gets files from LU it will get delta files also.
For LU I believe we keep 7 days of delta files. For SEPM you can customize how many days of delta files you want to keep.
The roughly 150MBs is full content size, not updates.
If a client already has content applied then only an update is needed not a full content revision. The updates are delivered either by SEPM, by GUP or by LU. All of these delivery mechanisms for content use delta form. These "updates" are generally around 200K to 300K.
Hope that helps you!!