For HD content using MPEG2 you can go to a maximum of 2 hours on a 25GB Blu-Ray disc and initially all HD-DVD and Blu-Ray content was on MPEG2. Now all the latest Blu-Ray titles are using MPEG4 to make room for more content like commentary, Blu-Ray Live, and other additional features. HD-DVD as you know is dead. Moving to MPEG4 has given room for twice as much content i.e. 4 hours of video or rather 2 hours of video plus other stuff which is what an average Hollywood movie runs for (i.e. 2 hours).