Shrinking the size of Oracle Virtual Box

First, zero fill your virtual disk. Boot the VM and run:

sudo dd if=/dev/zero of=/bigemptyfile bs=4096k status=progress
sudo rm -f /bigemptyfile

Shut down the VM and VirtualBox.

Then on the host run:

vboxmanage modifymedium x:\path\to\disk.vdi --compact

Note: You'll have to specify the path of vboxmanage (generally 'C:\Program Files\Oracle\VirtualBox').