You've been messing about in your VM, installing and removing software, etc. Your disk space usage inside your machine says you've only used 3Gb of disk space, yet your actual image is much bigger. How do you fix it?
It's fairly simple to fix. First, you should probably remove all the unnecessary crap that you never use. There's a bunch of utilities out there that can help you with this.
Defrag your drive using your favourite defragger.
Now you will need one of two possible utilities (There may be more, but these are the ones I'm aware of):
Precompact.iso is MUCH easier to use. All you need to do is mount the ISO inside your VM, and it will prepare your disk for compaction.
SDelete is only marginally more difficult. Run it in a command prompt like so:
SDelete -c C:
(or use whatever drive letter you want to compact). SDelete will write zeroes to
the free space on your drive image. This allows compaction to take place
properly. Note – this is exactly what the Precompact.iso does, just without the
fancy Windows GUI progress bar.
As soon as Precompact or SDelete is finished, shut down the VM.
Open command prompt on your host machine, and navigate to the folder where your hard disk images are located.
NOTE: It's probably MUCH easier to have the path to Virtual Box set in your PATH statement, otherwise you have to specify the full path to VBoxManage every time you use it.
In the command prompt, run:
[path to virtualbox]\VBoxManage modifyhd "Name Of Image.vdi" --compact
Your disk image will now be compacted, and should end up quite a bit smaller than it was. If you get an error about the disk image not being found blah blah, specify the FULL path to the image, like so:
[path to virtualbox]\VBoxManage modifyhd "C:\Users\Username\.VirtualBox\HardDisks\Name of Image.vdi" --compact