Compact VirtualBox image:
To compact:
1. Open the virtual machine
2. dd if=/dev/zero of=/bigfile (wipes free space with zeroes)
3. rm -f /bigfile
4. Close the virtual machine
5. vboxmanage modifyvdi nameof.vdi compact
Compacting will reduce the size of the virtual machine. I compact my Current virtual machine often. I save considerable space each time.
FWIW: To compact a Windows virtual machine, download a utility called sdelete. Use that command to zero wipe all free space. Then compact the same way using the vboxmanage command.