Compressing Disks

• https://www.howtogeek.com/312883/how-to-shrink-a-virtualbox-virtual-machine-and-free-up-disk-space/

Windows Guest

• defrag
• get sdelete from SysInternals
• sdelete64.exe -z c:

Host

• vboxmanage modifymedium disk some-disk001.vdi --compact