How can I cut down on the size of iMovie Library.imovielibrary?
Is there a way to make it smaller?


Sure — in iMovie, delete what you don't need and the size of the library will reduce.

If you're looking for more detail regarding what is taking up space in your library, you can browse the library with something such as DaisyDisk1 which will show what is using the most space.


  I don't have anything important in iMovie - what happens if I simply delete it?


You'll lose all your media from your iMovie library, including footage, projects and associated files such as render files (although if you don't have the media any more the render files are pretty useless).

Your iMovie settings aren't stored in the library, unless the settings are specific to the library or something in the library, such as project settings.

1 DaisyDisk needs a bit of prodding to look in packages — right-click the package and choose Show Package Contents.