Scenario:
VirtualBox version = 4.0.12
Guest OS = Win 7
Host OS = Linux Mint 9 Isadora (based on Ubuntu)
Perform these tasks on all the Win 7 local hard-disks:
- chkdsk /F
- defrag /X
- sdelete -z
Shutdown the Win 7 VM, then run this command on Mint:
$ VBoxManage modifyhd win7.vdi --compact
That freed up 5GB of space for me, a 20% reduction.