Apparently it’s safe to throw away the ‘Render Files’ (and sub-folders) from iMovie projects:
Yes, and iMovie can even delete (all) of them for you.
Go to iMovie > Preferences and click “Delete” next to Render Files (last item on the bottom for me).
I’ve only used iMovie a handful of times, and frankly, that was too many. It gets the job done, but it’s inexplicable and bloody-minded. Here’s an example:
What happened was that I tried to import a movie, but I inadvertantly picked the wrong one. But I had lots of time to rue my error, staring at this dialog. Because, you see, iMovie doesn’t have a “cancel” button. Arrgh! That’s forgivable in an application that gets busy and makes you wait 2-3 seconds for something, but when it wants you to cool your heels for 10 minutes at a stroke, not having a cancel button is ridiculous. From the HIG:
As much as possible, allow users to do whatever they want at all times. Avoid using modes that lock them into one operation and prevent them from working on anything else until that operation is completed. … If an application uses modes, there must be a clear visual indicator of the current mode, and it should be very easy for users to get into and out of the mode.