My iMovie library is about 50GB, and appears to be one file. Does time machine version this file after each change, or is it intelligent enough to see into the library, even though I can't?
All Apple iLife and iWork apps are optimized for storage to make Time Machine run well.
When you see a 50 GB library in finder, you can control click and show package contents. The "library" is a highly ordered collection of files, so when you are editing clips and effects and assembling a project, only the small changes you make are backed up each time. The large clips don't change as you add them to a project and transform them.
Be sure you don't move/ trim / edit / delete any files stored inside a package directly. That will usually break the program and require you to retrieve a backup where the database that tracks all the component files matches the files you actually have on disk.
If you're interested in the command line - the
tmutil compare option is handy. As soon as a backup finishes, you can run that to see the files that change often (there are tons). Then you could open iMovie (and only iMovie) and make a small change - then rerun the
tmutil compare to convince yourself that small file changes mean small incremental writes the next time a backup starts.