Mac storage basics · Photos, music and video
Where is the iMovie library and why is it so big?
Short answer: It's the iMovie Library package in your Movies folder and holds copies of everything you've imported plus render files. Delete old projects and events from inside iMovie.
iMovie stores projects, imported media and render files in a package called iMovie Library in your Movies folder. Imported footage is copied into it, so it can be as big as everything you've ever imported.
Delete projects and events you no longer need from inside iMovie rather than in Finder. Finished work can be exported to a single video file and the project removed. You can also move the whole library to an external drive with iMovie closed and open it from there.