Has Warner finally started using format-specific encodes?

[JBlacklow]

While browsing er...certain corners...of the series of tubes, I came across evidence of the size of the HD DVD versions of Harry Potter 2 and 4 and Blu-ray versions of Harry Potter 3 and 5. Apart from the weird numbering/even-odd versions, there's a pretty big discrepancy in size:

Harry Potter and the Chamber of Secrets HD DVD (2h41m): 25.16 GB
Harry Potter and the Prisoner of Azkaban Blu-Ray (2h22m): 39.46 GB
Harry Potter and the Goblet of Fire HD DVD (2h37m): 28.09 GB
Harry Potter and the Order of the Phoenix Blu-Ray (2h18m): 47.79 GB

I'd love to believe that Warner has suddenly seen the light and started using encodes specific to the bandwidths of each format on these titles, but I'm still doubtful. Could I be missing something? I know that on Blu-ray, all five films are using PCM and that OOTP has some extras in 1080p (as opposed to 480p on HD DVD), but would that really account for a 15-20GB difference, especially considering the longer length of the HD DVDs?

EDIT: Added runtimes.