Why don't DVD's have a standard "density"

[Bobby Digital]

By density 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.

[setarip]

See my response to your other (Duplicate) post...

[ashy]

I think you need to a look around the net for info on bitrates.
Just because one 90 min movie turns out a certain size it doesn't mean all will be the same size.
Different movies require different bitrates.
Usually movies with lots of action will require higher bitrates and hence will result in a larger file size.