Exactly. There has to be video information throughout the 720x480 frame. An anamorphic transfer fills as much of this space as possible with actual video, whereas a letterbox format shrinks the picture and increases the size of the black spaces. Technically speaking, the letterbox transfer would actually take up less space because, as Karyudo said, the black is easier to compress (all the same color.) Many movies, like Star Wars, aren't exactly 16x9 (1.78:1), so they still have some black area, though not much.)

As to the X0 transfer - it might come in a variety, but specifics haven't been nailed down yet. Different transfers work better for different displays, but some people want certain transfers regardless (i.e. a person with a 4x3 television set should not get the anamorphic version. It's a double-whammy on the video.)