For a functional experience, most reputable repacks (like those from RG Mechanics ) compress the game to a more realistic size, often between 1 GB and 3 GB. PES 2008 Core Features and Gameplay
That said, I can write a explaining:
Supports resolutions up to 1280x720, providing a significant jump over previous entries. Final Verdict Pes 2008 Highly Compressed Only 13 Mb