Best way to offload a FCP Library

As Tom says, it's best to delete all files once you're done editing. By eliminating large and unnecessary files, backup copying time will be shortened.

Even if you delete the files, some files will remain, so please delete them manually. You can further reduce capacity. *Uses translation function

I noticed a slight ambiguity in the answers you were given.

So to avoid even the smallest confusion for anyone reading this thread, the references to deleting all files meant all rendered (or generated) files.

Highlight your Library and open File in the menu bar . . . then click Delete Generated Library Files.

In the window that opens, select everything as in the picture below and click OK.

Yes you should delete all the files. They will take a great deal of space. As long as you have the original media the project will be fine. The generated files can be rebuilt at any time.