Here is the technical breakdown of what happens inside a Halo 2 Highly Compressed installer:
While data compression is real, the laws of physics and information theory apply. Compression algorithms (like ZIP, RAR, or 7z) work by removing redundancy. A game like Halo 2 consists of high-fidelity audio, high-resolution textures, and pre-rendered cutscenes. These file types (MP3, BINK video, DDS textures) are already compressed. You cannot compress an already compressed file significantly without losing data. halo 2 highly compressed
Project Cartographer is a fan-made patch that: Here is the technical breakdown of what happens