The reason there is more stuff on a Bluray disc is capacity. A regular dvd has 4.7GB (single layer)8.5GB (dual layer).Approximately 3 hours of record time on a dual layer disc. An HD disc has 15GB (single layer)30GB (dual layer)51GB
(prototype triple layer).approximately 13 hours dvd quality or HD quality 5.1 or 3.3 hours depending on encoding method. Bluray has 25GB (single layer)50GB (dual layer)100GB (prototype quad layer),approximately 23 hours dvd quality or 8.5 or 5.6 hours, depending on encoding method. So the reason Bluray has more stuff on it is because they can. If the HD format had more space they would put more stuff on it. Both HD and Bluray have the same audio specs and the same resolution so there is no difference there. So it really has to do with the capacity, bigger is better. And also on Bluray the extras are in the same format as the movie ,not in lower dvd quality like on the HD dvd, due to lack of space. shawn