When you delete files in your virtualbox VM in order to free up space on the host filesystem, this space is not automatically reclaimed.
In order for the host system to see the changes you need to rewrite the free space with zeroes.
Follow the below steps to perform this operation:
Install zerofree package. It is needed to rewrite the free space with zeroes.
Mount the filesystem as "readonly". This is needed for the tool to be able to perform it's task. If you're working with the "/", easiest way to mount it as readonly is to edit the kernel parameters.
