iMovie 10 stores all your Events (as well as your Projects and other related files) in the iMovie Library.
Right-click on it and choose Show Package Contents and you can see what's inside.
However, you shouldn't move, rename or otherwise change the contents of the iMovie Library in the Finder or you can really mess up iMovie.
If you want to delete Events or Projects, do it from within iMovie.
Thanks for the response, Matt. I did just find out I could show all the rejected clips in imovie. I selected all of them and deleted them. The imovie library file now is shrunk to 2.93GB. Still mysteriously large but a lot better than before. Any idea what those 2.93GB would represent and how I can get rid of that? Thanks.
A couple of thoughts....
When you convert your projects and events to iMovie 10 from iMovie 11, you have the option to keep your iMovie 11 Events and projects intact so you can continue to use iMovie 11.
If you took this option, it uses hard links so that both iMovie 11 and iMovie 10 can access the same clips but not take up double the space.
This has other implications:
1) If you delete your iMovie 10 event, it does not save you any space. You have to delete both the iMovie 10 and the iMovie 11 event to get back the space.
2) Hard links do not work when you copy to a new drive, so if you copy your iMovie 10 and your iMovie 11 events over to a new drive, now they will take up twice the space.
3) Therefore, if you are thinking of moving to a new drive, you should move the iMovie 11 events and projects over (from within iMovie 11). Then open iMovie 10 and convert the iMovie 11 stuff to iMovie 10, and it will use hard links and not take up double the space.
Hard links are also used in Time Machine so you can have the same file assigned to multiple dates without taking up more room. Also used in Final Cut Pro X extensively. For example, you can import your iMovie Events into FCP without doubling up space.