Modern Warfare has been out for almost two years, and after continuous updates, the game takes up a huge amount of space. Here's why Modern Warfare's file size is so huge. Modern Warfare started life with a massive 175GB requirement to store on drives, making it one of the biggest games ever.
The increased file size was "due to the increased amount of content" at launch, including map files for Warzone, as well as the Zombies mode, and the fact that a lot of content from Modern Warfare II carries over to Modern Warfare III.