time to compress

[kdwoell]

I have known for years that for a given frame size, it takes longer to compress digital video at a higher data rate than it would for the same digital video at a lower data rate. What I would like to know is technically what is going on?
I work with streaming media a lot (Real and Windows) typically starting with full frames, uncompressed .avi or quicktime files and use something like media cleaner pro to compress the files.
Does anyone have some thoughts on this?