Call of Duty: Modern Warfare 3 is a proprietary title owned by Activision. Downloading a full, uncompressed copy without purchasing it from an authorized retailer (Steam, Battle.net, or physical disc) is considered piracy. Highly compressed repacks fall into a legal gray area; while the compression technique is legal, distributing copyrighted game files without a license is not.
If you are looking for the original 2011 title, it is significantly smaller and more "compressed" by design: Storage Required: Official Source: Available on the Steam Store selectively uninstall specific game modes to save space on your SSD? Call of Duty: Modern Warfare 3 is a