iMovie stores every imported clip and every generated render file inside a single .imovielibrary package. A few old projects can easily take 50-100 GB.

Short answer: if you are done with all iMovie projects, drag the iMovie Library.imovielibrary file from ~/Movies to the Trash. To keep projects alive, delete render files from iMovie settings or consolidate the library to an external drive.

Nuking the library entirely

If you have already exported every video you care about:

  1. open Finder
  2. click Movies in the sidebar (or press Command + Shift + H, then open Movies)
  3. find iMovie Library.imovielibrary (purple star icon)
  4. right-click and choose Get Info to see the size first
  5. drag the file to Trash and empty it

Next launch, iMovie creates a fresh empty library. Nothing else on the Mac is affected.

Keeping projects, removing the bloat

If you still need the projects:

Delete render files. These are the heaviest chunk inside the library and regenerate automatically.

  1. open iMovie
  2. click iMovie > Settings
  3. next to Render Files, click Delete

That is safe — iMovie rebuilds render files the next time you open a specific project.

Consolidate to an external drive. If the internal drive is full but projects must stay alive:

  1. plug in an external drive
  2. in Finder, drag the library file onto the external drive
  3. double-click it from the external drive to make iMovie reopen it from the new location

From then on, iMovie reads and writes directly on the external drive, freeing internal space.

Better next routes

If the drive is still heavy after iMovie is tidy, continue with How to Clear Adobe Premiere / After Effects Media Cache.

For the broader framing, use How to Find Large Hidden Files on Mac.