Compacting virtual disk image (VirtualBox)...

Scenario

Host OS: Manjaro Linux

Guest OS: Kali Linux

Action on guest OS:

- Log in to guest OS

- Clear package manager cache

- Uninstall unnecessary apps

- Delete unnecessary files

- Clear logs

- Boot into vm via Linux Live USB iso

- Open terminal

- Install zerofree if not exist

- Run the command below

$ zerofree -v /dev/sdX

- Replace sdX with the actual partition like sda1, sda2 etc

- Shut down the vm

 Action on host OS:

- Open terminal and run VBoxManage command

$ VBoxManage modifymedium disk /path/your-vdi-image --compact