windows server backup 2008 R2 - what is generating all the change data?

view story

http://serverfault.com – We have a small relatively idle windows server 2008 R2 installation that does basic filesharing and exchange for about 10 not very active users. When running a windows server backup, the incremental data daily is about 20GB. This is not coming from users shared files, nor from changes in their mailbox sizes. The total size of the installation is 249GB, which is mostly old files. Where is all this data coming from, and how can I reduce it ? Using online backup of the vhd file from the backup is taking a while because of this daily change. Is there some way I can at least see what files ar (HowTos)