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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.
The file size of Call of Duty: Modern Warfare 3 is well over 200 GB on consoles and PC due to a variety of content install packs, including Warzone files.