Quicktime 10.3, converting clips, and eating memory

When I clicked on one of the Clips that were stored as part of each old iMovie project, Quicktime had to convert it, which took a lot of time. (I have since learned about the conversion to 64-bit, etc.) I don't necessarily have a problem with this, but my more immediate problem is that after opening half a dozen random clips and letting them convert, I lost about 30 GB of hard drive space - even though I didn't save a single one of the converted files. I opened one more and watched the "XYZ GB available" number at the bottom of another open finder window drop by several more GBs. What is causing this and how can I recover that space on my hard drive.

Am currently running a batch of file conversions so I can't swap operating systems to check on this today. However, the basic answer is that one or more of the converted files is probably being stored in your system's default QY X conversion and screen recording location. Under Yosemite when you close a player window containing a conversion or screen recording and opt not to save the data, the associated "temporary" file is immediately deleted. However, it is possible that your Mavericks system is either programmed differently or your specific workflow somehow prevented the immediate deletion of one or more "temporary" files. Normally I would expect the tile(s) to eventually be automatically deleted during certain routine system clean-up operations but the "temporary" movie and associated data files can be accessed manually if you wish at the following "user's account" location:

~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Autosave Information

The easiest way to get there is by using the Finder "Go" menu with the "Option" key depressed to select the current user's "Library" folder and then proceed to the "Autosave Information" folder. Once there you can either use the Context Menu to open the "Unsaved QuickTime Player Document.qtpxcomposition" package file to access individual "temporary" files or delete the contents of the "Autosave Information" folder to delete all "temporary" movie files and associated data.

I went into the library to that location, but the autosave folder was empty. So now I'm really stumped I've also completely shut down and restarted, but no luck there either. Perhaps the next step is upgrading to Yosemite and seeing what happens...

Based on your latest post, I might try two other things. The first would be to scan storage for the latest files created. If you accidentally saved a converted file that might explain the missing available file space and determine where it is located. The other would be to run the Disk Utility (or similar) app to check disk/permissions (and/or rebuild the table of contents if needed). The sudden loss of disk space could indicate anything from normal HDD cache operations by certain apps (which would likely clear itself over time) to an indication of impending HHD failure.