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.
This is the case for Call of Duty: Black Ops Cold War as well. The entire game takes a whopping 125 GB on a person's hard drive, which is simply ridiculous since developers should be responsible for optimizing their games on PC.