The HD-DVD discs are both the same physical size as a DVD and a CD (5" in diameter). The problem is that HD video is too big to fit any worthwhile amount in the 4.7GB of space offered by DVD -- both HD formats are increasing storage capacity at least 300% to make room for the higher resolution of HD content (Blu Ray actually offers five times as much storage capacity per layer). Sure, you can compress it more and make it fit, but then you'll have lots of really crummy looking lines instead of fewer good looking lines. Think of it this way: why didn't Video CD's do 480i resolution ten years ago? Because it would have required so much compression that the final product would have been worse than a lower resolution.

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