It depends a bit on how much RAM you assigned to the VM. If you assign 5GB of RAM to the VM then Fusion will create a disk file of 5GB on bootup to back that RAM. There are vmx settings to turn that feature off, but it isn't recommended or supported.
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I'm just wondering if you did an upgrade to Windows 10 from 8.1.
If that's the case you can reclaim a good portion of your hard-drive space back by using the Windows disk clean-up utility to remove Windows 10 installation files (which can take up to 10Gb of space).
Then go to that VM's General Settings and there should a button to reclaim that space back for OS X (I'm unsure of the exact settings as I'm not in front of my Mac).
Of course, if you do this, (and providing you can do this) you won't be able to rollback to Windows 8.1 if you have any major problems with Windows 10.