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*

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

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

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