The developers confirmed exactly what
@KeyboardDemon and I said. They said in previous interviews, PR statements, that the reason why Warzone/Modern Warfare is so big, is because of the texture assets and the code. They said they were working to reduce the size in Next Gen consoles, but even then, the reason why the game is so crisp, is because of the texture packs (you download texture packs across all platforms, so it's not just exclusive to PS5). Hell, the PC version requires a hefty texture pack download all on its own. The other 20% of the texture pack is the Ray Tracing.
As far as the glitches go, they confirmed (go into that video, it's in there) that the integration caused bugs to pop up on another side of the game, then when they fixed the bug, it pops up over there, it's just a mess.
I understand your pain, but we were there at launch. PS4 version takes up 150+ GB on my hard drive. [As I said, Call of Duty: Warzone was bundled with Call of Duty: Modern Warfare.] You are currently getting off easy.
Oh, and
that reminds me: Call of Duty: Modern Warfare and Call of Duty: Warzone shares the same engine, the same assets. Caldera and Rebirth Island is different from the Modern Warfare disc, though.