Please explain compression sizes to me

[buffotoad]

Please explain why movies of somewhat equal lengths have very different compression settings in DVDShrink re-author mode.

For example, Intolerable Cruelty is 1hr 39min, and it needs to be shrunk 82.5% (movie only, 5.1 sound only).

On the other hand, Second Hand Lions is 1hr 48 min, and it needs no compression (same settings - movie only, 5.1 sound only).

How come?

(I'm new at this, but I don't think I have made a mistake with the settings in DVDShrink.)

Thanks.

[rcb]

From the glossary:

Bitrate
Bitrate or Bit Rate is the average number of bits that one second of video or audio data will consume. Higher bitrate means bigger file size and generally better video or audio quality while lower bitrate means lower file size but worse video or audio quality. Some bitrate examples in common video and audio files:
MP3 about 128 kbps (kilobits per second)
VCD about 1374 kbps
DVD about 4500 kbps
DV about 25 Mbps (megabits per second).

[djmattyb]

Variable bitrate is what makes the difference. Say you have two movies (A and B). If movie A has lots of movement it will need a higher bitrate. Say movie B has a lot of still camera shots where it's just a person on the screen and the background isn't changing, it won't take much to compress the frames. The way the compression works is it looks at each part of each frame. Whatever is the same, it doesn't have to compresses that part over and over because it's the same for each frame, it only has to 'redraw' what is moving.

Or something like that.