Sharing a file in HD from iMovie 10.0.1

When I finish editing a project in iMovie 10.0.1 and want to share it as a file, Share/File lists four options: SD 480P, Large 540P, HD 720P and HD 1080P. However, I am only allowed to choose SD 480P - the other three choices are grayed out. Any thoughts/suggestions? (I'm running OSX Mavericks 10.9.1.) Thanks in advance!