I got the same problem and I finally found the solution. The cause of the problem might be that I created only one project file. Then I created a video clip and export the clip. After the job is done, I deleted all the raw data files and create a new video clip again and again. Over and over. Even though I have deleted all the raw data files but it seems like iMovie still keep it and never really delete it from the database.
The solution is to create a new project file and delete all the old project files. That's all that I did to fix this problem. If this doesn't work, then go to "movies" folder. You will see the project file (the one that has "film" icon). Delete it. Back to iMovie and create a new project again.
I hope this comment can fix your problem.