Different types of files vary greatly on how much they can be compressed, and what compression methods are effective.
A game is different material to a movie. A movie has already had considerable data saving processes applied to it - typically sound is highly compressed, frames are stored as key frames & difference data etc. There's not much redundancy for a lossless (which is what you want) process to work with.
Commercial software which claims to be able to achieve huge compression incorporates numerous different algorithms, and analyses each file to decide what compression system to use. Naturally they don't give away anything about just how they achieve it (assuming their claims are valid).
Edited by Platypus, 24 October 2010 - 04:39 AM.