You may notice in iMovie HD that, after emptying the Trash, the disposal of clips did not appear to increase the amount of free disk space available to you. This can happen if you split a clip in the timeline and then move one of the split clips to the iMovie Trash.
As iMovie HD features non-destructive editing, this is normal behavior. Non-destructive editing means that the video clips you see and work with in the program are merely representations of the video files that reside on your hard drive and that you can modify these representations and not affect your originals. Non-destructive editing makes it easier to undo or alter previous changes without having to reimport video from your original source.
When you split a clip into sections, iMovie HD doesn't really split the actual file that resides on your hard disk—rather, it creates references to the original clip's footage and these references are what you are editing. The entire original video clip remains on the disk for usage until all references to it have been deleted from iMovie HD's timeline and Clips pane.
This means that until you move all the references that point to the original clip to the iMovie Trash, you won't be able to recover the expected amount of space because other items may be relying on this clip. By themselves, references don't take up significant space to recover the amount indicated by the iMovie Trash when deleting these items individually.