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.

Windows has a game mode that optimizes the system for maximum performance while gaming. Windows 10 and Windows 11 offer a killer Game Mode that can help make unplayable games playable  sometimes. It's especially handy in lower-powered systems that have lots of programs running in the background.

Overall, it's best if you keep Game Mode on. It will neither improve nor decrease your PC's gaming performance, but it will help your computer run a few degrees cooler and prevent annoying updates and notifications while you're playing, which should make your gaming experience more peaceful.