Name one other time where a modern sequel released this soon after the first game, and using the same engine, was triple the size of its predecessor. I'll be waiting.
The reason it didn't switch engine is because UE4 is still industry standard and switching to UE5 isn't viable - it's not ready yet and just put its foot out of the door. It wasn't ready when they started development on this. The real change here, if you bothered to look at what you're talking about at all, is that it's a generational leap. The first game had to also run on PS4 and Xbone (consoles from 2012) and conform with their capabilities. This is the primary factor for the upgrade here, not the engine change. UE4 even supports mobile phones, and mobile games definitely don't have file sizes of 43GB so your claim about the engine doesn't come into play at all. It's just a display of complete ignorance on your part about how games are developed, how platforms work and how compression and optimization is used.
That aside, we're talking about a four year difference here. Here's your examples, since apparently you're living in a cave.
God of War (40 GB) - God of War: Ragnarok (107 GB) 4 years
Vermintide 1 (30 GB) - Vermintide 2 (105 GB) 4 years
AC Odyssey (46+ GB) - AC Valhalla (160 GB) ~2 years
COD: WWII (90GB) - COD: Black Ops Cold War (175 GB) 3 years
Once again, the new game will most likely have more audio to work with, more textures, more meshes and larger map sizes than a 2012 console would have been able to handle. And all of this at a higher fidelity.