Data compression is a legitimate and fascinating field of computer science. Using algorithms to reduce file sizes is standard practice; "zipping" a file can significantly reduce its size, especially if the files are repetitive or uncompressed raw data. Modern game repackers (groups that specialize in compressing games for distribution) can shrink games impressively. By removing multilingual audio files and compressing textures, a 50 GB game might be reduced to 30 GB.
Do not search Google for random "FreedownloadDarksiders.exe" sites. Stick to names with a reputation in the scene (usually found on archive.org or dedicated gaming forums). Popular (Historical) Repackers for Darksiders 1: Mr DJ, BlackBox, RG Mechanics, FitGirl (Note: FitGirl’s version is usually around 3.8GB, very reliable).