When an iMovie project is being edited you are only creating a series of instructions telling iMovie what to do with the original clips, photos, audio tracks etc. when you share / export / render (different words for the same thing) the contents of the project timeline to produce a single self-standing video file.
Once you have created this video file it is independent from iMovie and doesn't depend on the iMovie project project contents any more so you can delete the project unless you want to use its contents again in the future to create a different or updated movie. Final video files are more compressed than the project contents so takes up much less space. However the final video file is only good for playing and not for editing.
To save space I would recommend using multiple libraries and archiving offline those that no longer contain projects that you are still editing or events that are required by current projects. As a minimum I would always keep an offline backup of your original clips and still photos.
Geoff.