Defragmentation and resizing to minimize HD size of a Virtual Machine Drive for Virtual Box

When creating a Virtual HD for Virtual Machine in Virtual Box, you can create a large disc space but the actual size on the disc can be much smaller until you fill it up.
But after you fill it, and then delete and try to free space the size on the host disc still remain large.

Here is how to resize the Virtual Drive HD on the host machine:

You have to do the following steps:

Run defrag in the guest (Windows only)
Nullify free space:

With a Linux Guest run this:
dd if=/dev/zero of=/var/tmp/bigemptyfile bs=4096k ; rm /var/tmp/bigemptyfile
Or:

telinit 1
mount -o remount,ro /dev/sda1
zerofree -v /dev/sda1
With a Windows Guest, download SDelete from Sysinternals and run this:

sdelete.exe c: -z
(replace C: with the drive letter of the VDI)

Shutdown the guest VM

Now run VBoxManage's modifymedium command with the --compact option:

With a Linux Host run this:

vboxmanage modifymedium --compact /path/to/thedisk.vdi
With a Windows Host run this:

VBoxManage.exe modifymedium --compact c:\path\to\thedisk.vdi
With a Mac Host run this:

VBoxManage modifymedium --compact /path/to/thedisk.vdi
This reduces the vdi size.

https://superuser.com/questions/529149/how-to-compact-virtualboxs-vdi-file-size