VirtualBox

1. Run defrag in the Windows Guest
2. Nullify free space:

Linux Guest

sudo dd if=/dev/zero | pv | sudo dd of=/bigemptyfile bs=4096k
sudo rm -rf /bigemptyfile

OR

telinit 1
mount -o remount,ro /dev/sda1
zerofree -v /dev/sda1

Windows Guest

cleanmgr
del C:\Windows\SoftwareDistribution\Download
del C:\Windows\Temp
del C:\Documents and Settings\<user>\Local Settings\Temp
del C:\Windows\$Nt*Uninstall*

rem http://cdn.chorke.org/soft/winx/sdelete/sdelete.zip
sdelete.exe c: -z
sdelete.exe d: -z

Compact

VBoxManage modifyhd xp-msql-box.vdi --compact
VBoxManage modifyhd xp-base-box.vdi --compact

OR

VBoxManage.exe modifymedium --compact xp-base-box.vdi 
VBoxManage modifymedium --compact xp-base-box.vdi 
vboxmanage modifymedium --compact xp-base-box.vdi

Knowledge

sudo apt install -y cpu-checker
kvm-ok

virtualbox before start
sudo systemctl stop    libvirtd.socket
sudo systemctl stop    libvirtd.service
sudo systemctl disable libvirtd.service

sudo systemctl enable  virtualbox.service
sudo systemctl start   virtualbox.service

References

Windows PowerShell 1.0 for Windows Server 2003 & Windows XP Windows PowerShell 2.0, WinRM 2.0, and BITS 4.0 How to compact VirtualBox's VDI file size? How to optimized Windows XP size? Creating a Windows XP Vagrant box Discover Vagrant Boxes Vagrant vs. Terraform Creating a Base Box A list of base Boxes Example Vagrantfile	Vagrant vs. Docker Vagrant Windows Getting Started Compact VDI Qemu