a couple of films are too big to fit on a hd-dvd disc. they can and do make use of the space, either a higher bitrate on the video or the the audio - 7.1 lossless PCM takes up a lot of space. think, 70-odd minutes of 2 channel pcm takes up about 620mb. a 2 & 1/2 hour film, ~1.24gb. 7 channels instead of two, ~4gb, plus the 0.1 channel. thats at the standard 44.1khz/24bit sampling rate as well., double it if you want to move up to 96khz/24bit. and then there's the DD TrueHD audio track, DTS: MA if its in there, standard DD/DTS, throw in a couple of commentary tracks..... it takes up a lot of space.
most of the time though, films released on both formats tend to be exactly the same, but anyway...blueray, and hd-dvd quality, wont be done on a dvd any time soon lol