If your virtual vm storage is getting less and you need extra storage space, you need to the the following :-
a) power down your virtual machine
b) add a new virtual disk (vdi) and store it at a specific folder in your host machine
c) power on your virtual machine.
d) goto disk management and you will see a prompt asking you to acknowledge the new disk partition
e) assigned a drive to it and proceed format that drive.
That's all you need to do to add a little extra space to your virtualbox.