Yes. Most likely the VM's virtual disks once had files on them that have since been deleted. If so, even though the quest OS is no longer maintaining pointers to the files they are still present on the disks (just like if you were dealing with a physical hard disk). vBackup creates a backup of each virtual disk at the virtual hardware layer. This is similar to cloning a physical hard disk. Basic compression removes the "white space" as the backup copy of the disk is created. Basically creating a thin-provisioned clone. Since the free space, as reported by the OS, is really no longer "white" it is maintained in the backup copy.

You can re-zero the unused space on the virtual disk using SDelete from Sysinternals (https://technet.microsoft.com/en-us/s...). This should greatly reduce the size of your backups.

Just to be safe, please be sure to make a good backup of your VM's before you run SDelete or maybe try it on a restored clone first.