Kwag, I think I have an answer to your previous post!
I have just read the inside jacket of my Pearl Harbor DVD.
It is explained that in order to be compliant with THX standard, the MPEG2 stream quality of the movie have been compared to the original master on critical scenes.
I assume that they have adjusted the MPEG2 bitrate so that no artifact and blocks show up during fast action scenes, hence a better "compressibility". LOTR was not so well optimised...
So THX-Video = compression bread and butter.
By the way : I could NOT fit Pearl Harbor on 1 CD with kvcd-LBR-Q20 (2 blocks overscan, subtitles at 85% of their original size, 3DConvo with your parmeters = 830 Mo without sound)

Any idea?