Client Backup Sized

Hello, the following may have been discussed somewhere but so far I have not been able to find it.

I just upgraded my HP Proliant microserver from WHS v1 to WHS 2011, I upgraded my data drives and thought it would be the time to try out the new version.

With WHS V1 my backups for all my client computers (full drive backups) never seemed to exceed 450Gb. With my first backups using WHS 2011 I have already filled up over half the data drives.
I am a server admin at work and I understand about backing up the full drive as opposed to just the critical data files takes more room, but it seems like the backup is copying the entire partition as opposed to just the data filling the drives ?

I understand that WHSV1 will no longer have official support after January 2013 and I like the idea of moving to WHS 2011 as it is based on Win server 2008 R2 but if it is going to fill up my data drives like that I just might have to go back. The loss of Drive extender didn't really faze me as I am running my data drive Raid 1.

Any ideas, solutions for this ?

Thanks in advance
HP Proliant Microserver, 4Gb ram, 1 x 160 Gb Sata OS drive, 2 x WD Red 2 Tb drives in Raid 1 for data.