window server 2008 backup always include c disk?

I have a server 2008 with 3 partition.
The fist (c) has 20 GB ofa data, the other only few GB
If I backup D where I have 1Gb of data, choosing custom option, deselect
system recovery, deselect c, select only D then the backup is always bigger
then 20 GB even if the backuped volume has few GB.
If I try to restore I can always see and restore files from D
Why it takes >20GB??
Is possible that old data deleted from D are still visible to window backup?
CHKDSK show only 1 GB of used space.
Any Idea?
Thank you
Alessandro