report is showing my backed up data is 11GB, but on disk it is only 3GB

Folks: I am new to Networker, but have been pushing through with learning it. Currently I am using 7.6 and I have a simple job created, which only backs up user folders on a Windows 7 Pro machine. The folder shows as 3GB on the HDD; however, after I successfully run a server initiated backup against the folder - my reports are telling me that it takes up 11GB on tape. Why is that? I only have the folder selected - and since I am not using VSS I do not have the 3-5  VSS items selected as well. Thank you.