1. Open the disk properties and select Clean Disk (there may be no option to clean system files).

2. Disable hibernation, open CMD as an administrator and run: powercfg /h off.

3. Turn off virtual memory in advanced system settings.

4. Run CMD as an administrator and run the following command:

Compact.exe/CompactOS: has always been like this

5. Use the SDDelete command to determine the unused space. Note that it is especially important to use the -z parameter. Other parameters will increase disk space.

sdelete.exe c:\-z

6. Use the VBoxManage command to close the client and compress the VDI file.

vbox manage modify media disk/lab/biztalk 20 16-compact

7. Operation completed

[reference]

Compress the Windows 10 virtual machine and shrink its VDI/VMDK disk image on the VirtualBox host.