Well - I don´t think just taking normal filebackup of the VM server is the right approach. Backups are fine, but apparently Urbackup recognizes the size of the VM-disk as nominal value contrary to the real (used) value, meaning it consumes way too much space than needed. One of my VM´s is moved from a Windows Server, and has a virtuel disk attached with a 120GB volume - (because it was created from a physical machine) - but the real used space in the virtual disk is only around 50 GB - but it consumes 120 GB of my backupspace.
The total size of a full backup of the VM-Server is 290 GB - while the actual use (df -h) is 12 G, so …I think I´ll just have to exclude the subdirectory which holds the VM-disk images, and find another way of storing those by rsync to another physical server.