Pretty strange. I have a 16x9 PowerPoint presentation. In presentation mode it takes up the full screen on my laptop. However, when I hit F8 to project (even with no projector connected) the image is reduced and there are black bars all the way around the image.
Choose Slide Show | Set Up Slide Show
Under "Resolution" verify that it's set to Use Current Resolution or a resolution that matches your screen rez.
Sounds like it might be set to some other resolution.