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Pedz, lots of issues come into how big a HD film ends up being. Video codec choice, video bit rate, audio codec choice, audio bit rate, length of the film, etc. It's not just a simple "oh, well every film is only this big, so there should be plenty of space for extra features." As for the extra features, companies did the same thing with DVD when it launched. Give it some time. Plenty of Blu-Ray films are being released with extra features these days.
As for HD-DVD, their single layer discs were 15GB and their dual layer discs were 30GB. Most discs at the beginning of their launch used dual layer. Blu-Ray, on the other hand used their single layer 25GB discs. Also, the encoding methods and audio formats were different, as well. There's a lot more to it, honestly.