If you're struggling to find space for Call of Duty: Modern Warfare 3 on your Xbox, you're probably not alone. In fact, Activision has now issued a statement via social media addressing the game's file sizes across all platforms.
The reason it's so much "larger than last year" is tied to the "increased amount of content available on Day 1". This includes open-world Zombies, carry-over support for items from Modern Warfare 2, and the map files for Warzone. Activision has further noted how the "final installation size" will actually be smaller "than combined previous CoD experiences".
And if you are having trouble managing all the Call of Duty content, you can actually use the 'manage files' section in the Call of Duty HQ launcher menu to uninstall specific content, if you're not planning on playing all of it, or are done with certain parts.