If you are sure that both libraries are working (able to open and edit projects, share out movies, contain all of your media), and they both contain the same items, then you can delete the smaller library. Since one library is 10 GB larger they can't have exactly the same content. So I am assuming that the larger library contains everything that the smaller library has, plus 10 GB of additional data.
Before deleting one of the libraries open iMovie in the other library (the one you want to keep) and select the library's name in the iMovie side bar. Then do a File/Consolidate Library Media to make sure that all media is copied and included in the library. Then go to your Projects browser and select all of the projects as a group, and do a File/Consolidate Project Media.
Personally, for safety you might consider just keeping them both since you have an external drive. That would be your call, of course.
-- Rich