In the Finder (Movies folder), you can Control-click on the iMovie Library icon and, from the drop down menu, click on "Compress (library name)". That will create a compressed zip file of the library that you can archive. Try it first with a throwaway test library to confirm for yourself that all is good. It is too late to use this method on your present library, since you have altered it's content.
Never change or delete anything in the iMovie library unless you do it through the iMovie app itself. When you convert a clip you change it into a new clip that iMovie will not be able to find. You will need to re-import your original media to re-establish the links. Fortunately, as a workaround, you do have your exported final product that you can always re-import into a new iMovie project and do some limited editing if need be. Or, if your keep a Time Machine backup you can restore you library to a point in time immediately before you made the conversions to the Original Media.
The best way to store iMovie libraries is to store them in an external drive that is formatted Mac OS Extended (Journaled) or APFS. Then you don't need to worry about space or compressing.
-- Rich
