iMovie hard drive space
Try looking at your cache. Navigate to ~/Library/Caches/ and look for a folder for iMovie (may also be labeled com.apple.iMovie). Check its size. If it's quite large, then you've found the culprit.

Note: deleting the contents of Caches is safe provided the application isn't running. While not necessary, deleting the contents of the entire folder should prompt a reboot.

You may also elect to give Grand Perspective a try, which will show you the contents of your drive and how files are arranged on it. This will help hunt down the loss of space you are experiencing.