How does time machine handle iMovie libraries?
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.