Why are some DVD's bigger than others

[Bobby Digital]

By "bigger" I'm referring to the filesize:length ratio. ie. if a movie is 90 minutes long, it should be X megabytes.

I always burn using DVD shrink to re-author the movie to just the main video (no compression) and the AC3 audio track, I remove all menus, extra features, extra angles, extra audio tracks, and subtitles. I do all this because I'm a stickler for quality and it's the only way I can fit the whole movie with no compression.

With no compression I'm usually able to fit about 120 minutes of video onto a standard writable DVD. I've experienced though that with some movies, ie. Godfather trilogy, I can only fit about 90 minutes of uncompressed video using the same method.

My question is, why is this? Isn't there a standard resolution and framerate, thus creating a standard filesize to length ratio? I know it's a stupid thing to want to know, but I'm one of those types that needs to know everything. Any help would be appreciated.

P.S. I already posted this in the DVD Burning forum but I realized it's more of a general question so that's why I posted it here as well.

[setarip]

Although most Region1-NTSC DVDs have an indicated video bitrate of 9800kbps, that's actually the maximum. If your standalone DVD player provides the ability to view the bitrate (most Sony standalone players have this feature), you'll note that the actual bitrate varies widely during the playing of the movie.

Higher video bitrate = higher video quality - and I've got to believe Mr. Coppola insisted on his DVDs being produced with the highest quality possible.

On a MUCH smaller scale, you should compare the size of the .AC3 audiostreams...

[khp]

The thing is, you never burn a DVD without any compression. Otherwise a 2 hour movie would take up 2*60*60*29.97*480*720*2(NTSC) bytes or about 150GB , for the video alone.
DVD's always use mpeg2 compression for the video.
How much space a given movie takes up depends on a number of factors, Like how well the movie compresses, action scenes compresses worse, than scenes with little or no movement, and a high quality source compresses better than a low quality source.