According to Activision's post on X (formerly Twitter), the huge file size is due to the increased amount of content available Day 1,  which includes map files for Call of Duty: Warzone, support for Modern Warfare II items, and the new open-world Zombies mode.

Call of Duty: World at War is a 2008 first-person shooter game developed by Treyarch and published by Activision. It is the fifth main installment of the Call of Duty series and is the fourth entry in the series to be set during World War II.