Occasionally, these files might contain a "Game Demo" or a fan-made "GTA rip-off" that looks like GTA but is actually a completely different, low-quality game (perhaps created on an old engine) that has been rebranded to deceive the downloader. It might feature a GTA title screen, but the gameplay will be non-existent or purely superficial.
Most of these assets (Audio in MP3/OGG, Textures in DDS, Video in Bink/MP4) are already compressed formats. You cannot compress a file that is already compressed without losing data. Gta 5 Iso File-7z 24mb