Unlocking iMovie '09 & '11
I have seen some comment that the new iMovie is space-efficient. The idea is that your drive only needs to contain one copy of some footage, and all projects built on that footage will not needlessly multiply it like iMovie HD does. Others have commented that iMovie '08 will take a lot of hard drive space because it acts like an iPhoto for your footage, storing and cataloging it all so you don't have to leave it on external devices like DV tapes. Footage, just like photos, adds up quickly, just as quickly filling up your hard drive.
So which is it? Are we getting a fit Jack Sprat or his more corpulent wife? The truth is that it is up to you. There is certainly no requirement to leave footage in the Event Browser. It is up to you, however, to get rid of it. So if you select footage in your Event Browser and hit delete, is it deleted? Almost, but not yet. Here is how to delete a clip.
Deleting a clip in a project
I am just getting this out of the way before we move on to real, space-saving deletion. As mentioned before, iMovie edits by referencing source video, not copying it. This means if you delete a clip in your movie project, you have only deleted a reference to the clip. The clip is still on your hard drive, taking up space. You need to delete the clip in the Event Browser to finally be rid of it.
Deleting a clip in the Event Browser
The Delete key in iMovie '08 actually works in a very similar way to the Delete key in iMovie HD. Previously, if you deleted a clip, it went to your trash. There it lived indefinitely, that is until you emptied the trash, at which point it was gone forever.
iMovie '08 doesn't move clips to the trash. It just rejects them. A "rejected" clip is actually just a clip marked up by the Reject tool. This behavior is a lot like moving clips to the trash. You can read more about the reject tool in my post about the Advanced Editing tools.
How to use the advanced editing tools
The Delete key just marks a selected clip as a reject, but doesn't actually delete it. To accomplish that, you need to take an excusive look at all your rejects. You can do this by viewing Rejected clips only in the View menu, or in the pull-down menu in the lower left corner of your window.
Once you have selected to view only rejected clips, you have the opportunity to remove them from the Event Browser and move them to the trash in the Finder. (Yes, this is now a three-step deletion process. If you accidentally delete something from this point on, you better have a dang good excuse.) The iMovie window is pretty obvious about how this is done. (Click to enlarge)
What happens behind the scenes here is pretty cool. Once you move your rejected clips to the trash, iMovie '08 will actually split the source video file in the Finder into separate clips and and toss the rejected clip file into the Finder trash. It also does this with the corresponding thumbnail footage that it generated when you imported the video. To finally be truly rid of the footage, you have to empty the trash in the Finder.
Once you delete the footage in iMovie, by which I mean the second delete when you send it to the Finder trash, the surrounding footage that survived becomes two different clips, and that forevermore. You can always reimport the footage, but you can't resurrect it from the Finder trash and rejoin it to its surrounding footage.
I'll take the lean
So whether your iMovie is a Jack Sprat or a Mrs. Sprat depends on your diligence in deleting video from the Event Browser. I personally find that I have always recorded something that I will never bother watching again, so I expect I will be keeping my iMovie on a regular diet.