Rsync of LVM Snapshots copies whole file

[Peter P GMX]

Hello, anybody has a clue why this happens?

The scenario:
Each night I create LVM snapshot of my volume which contains some 
virtual disk images (VirtualBox VDIs). Then I rsync the content of one 
of the snapshot directories to a backup server. Each file size is 
between 1.3 GB and 6GB.
I run the following commmand:
sudo -u rsyncbackup rsync -avzP --exclude-from 
/home/rsyncbackup/excludefile -e ssh 
"192.168.0.3:/mnt/vdi_snapshot/VDI/mit_backup" 
"/mnt/crypteddevice/linuxX2/mit_backup"
The source directory is part of the LVM snapshot.
The destination directory is a truecrypt device which is already mounted.

What I would expect:
Only the differences of the virtual disk files should be transfered.

What happens:
I discover that always the whole file is transfered. Even if the 
original virtual machine is idle (so only some log files should have 
been enhaned in the virtual disk images).
I do the following:
- take LVM snapshot
- Rsync the desired VDI file (2GB are transferred), it takes 10min
- destroy LVM snapshot
- take new LVM snapshot with same parameters as before
- Rsync the same file (2GB are transferred), it takes 10min

Am I doing something wrong?

Best regards
Peter