To fix the problem, go into Fusion and bring up the “Virtual Machine Library”.
I’ll go through the same steps, and as you can see here, my 25GB virtual hard disk for Windows 8 is eating up more than the requested size.
Once it’s powered down you’ll still be running Fusion, you just won’t have a running Windows system. It’s not too difficult to remedy this – and remember most of the space used is empty and unused – but you do need to shut down the guest operating system before you proceed, so as a first step launch VMWare Fusion and choose “Shut Down” from the Windows OS. In other words, you might have set up Fusion to run Windows 7 on a 20GB virtual disk just to find out that the file allocated on the Mac side for the disk is more than 20GB in size, even quite a bit more. Occasionally, though, things get upside-down and the virtual disk space image can end up being bigger than the disk you’re mapping, which is really weird. Most of the time that’s not a problem and it just performs its trick behind the scenes without you ever having to worry about it, actually using less disk space than you’d think.
There are various things happening behind the scenes, but basically it’s smart enough to not allocate actual physical disk space for the free space, even though VMWare Fusion reports to the guest OS itself that the space is all available. VMWare Fusion and similar programs use interesting algorithms so that you can ask for a 20GB virtual hard drive for the guest operating system and yet it only takes up 5-10GB of space on your actual hard disk.