Final Cut Pro bundles every imported clip plus every auto-generated render and proxy file inside a single Library. That is why a 10 GB project easily turns into a 500 GB Library on disk.
Short answer: open the Library in FCP, then choose File > Delete Generated Library Files and tick Delete Render Files (All). Tick Delete Proxy Media only after export.
Why Final Cut libraries balloon
Unlike Premiere, Final Cut Pro copies source media into the Library by default and then writes two kinds of generated media beside it:
- Render files — background-rendered previews for effects, transitions, and titles so playback stays smooth
- Proxies — lower-resolution versions of the originals used while editing heavy footage (4K, ProRes RAW)
Both regenerate on demand. Keeping them between editing sessions is convenience, not safety.
Deleting generated files the safe way
FCP has a built-in cleaner — there is no reason to dig into Library internals through Finder.
- open Final Cut Pro and click the Library (four-star icon) in the sidebar
- in the menu bar, choose File > Delete Generated Library Files...
- tick Delete Render Files and choose All
- tick Delete Proxy Media only if the project is finished or you are fine editing at full resolution
- click OK
The same menu also works on a single Event or Project inside the Library — useful when only one exported video needs its cache nuked.
Better next routes
If the drive is still tight after FCP is trimmed, continue with How to Clear Adobe Premiere / After Effects Media Cache.
For a broader sweep of Mac creator bloat, read How to Clean Up Mac Storage.