FCP taking up lots of storage even after deleting render files

Delete all generated files, not just render files. You usually don’t need optimized media, of proxy. They can be recreated later in the unlikely case that they are needed. Also, in the preferences, uncheck the creation of optimized and proxy on import. That is probably how you got those large files in the first place.

Right-click on the library in the Finder and use Show Package Contents. Be careful not to move or alter any of the contents.

If you had Automatic settings for Analysis switched on during import there may be files still hovering in folders.

See here for details:

Render files deletion with 10.5.4

Al

If Finder Info says the library is 300 GB there are probably large media files stored inside it. You could go inside the bundle to see what’s happening in specific folders.