I don’t know if it is the files I have on my computer, but each of my machines’ index file size occasionally just goes out of control. This covered KB 2838018 The Windows.edb file grows very large in Windows 8 or Windows Server 2012. Unfortunately however even after the hotfix is installed, as seen below, 66 GB for the index on a 439 GB drive seems very much like the symptom described.
The instructions in the KB cover how to rebuild the index. I find just stopping the indexing services, deleting the file and restarting the services a quicker space recovering resolution. Note that your search everywhere in Windows, even applications from the start menu, won’t work for a little while.
Finding space to free and finding this rogue file is very easy with Space Sniffer, it is the best disk space visualization since Space Monger 1.4. Be sure however to run it as Administrator, otherwise those system files will not be shown.
The file is located at
C:\ProgramData\Microsoft\Search\Data\Applications\Windows. After a rebuild it is a more reasonable 1.8 GB.