A .vdi file (“VirtualBox Disk Image”) is at least a few gigabytes in size if you run a virtual Windows OS or Linux OS.
To reduce the filesize of this file, you can compress it. This can potentially save a few megabytes or even gigabytes on your disk. Maybe needless to say, but you want to close down this virtual OS first!
VboxManage modifyhd "E:\virtualbox\vdifiles\Ubuntu_x64.vdi" compact
You should replace the path with the path of your own .vdi file. This command will make your VirtualBox OS more compact.