The crazy thing is that a lot of today’s models are actually much more simple than they used to be. Look at comparisons for old games like Mario Galaxy versus Mario Odyssey for good examples. But this is done because it allows for more detailed environments; The lack of polygons means that (as long as the textures can be loaded fast enough) your rendering goes much faster. You’re not being bottlenecked by polygon counts anymore, because there are fewer polygons.

But this has caused texture sizes to balloon, because now you’re shifting nearly all the heavy lifting over to those textures instead of the polygons.

To be clear, the 4k textures are an optional free extra download that bumps the install size up significantly above the ~50GB. The 1024x1024 textures that come with it as standard will still have taken up a good chunk extra over 2004's version though, of course

I've dug around in the files a little bit for modding purposes and as far as I can tell, one of the biggest reasons for bloat is the way that they implemented an ethnicity system in the remaster. In the original, units just had a skin colour that was roughly appropriate for the historical home region of the faction they're from. In the remaster, they added a system that gives the units some variety in appearance that takes in to account where the unit was recruited; if you recruit soldiers in England, they'll be paler on average than the ones recruited in Algeria. However, it seems like the way this was implemented was to give every single unit a unique texture for every ethnicity. There's no common set of, say, five-ten faces for southern European men, for example. Every single unit has one southern European face, one northern European face, one north African face etc etc. Same goes for the visible skin on the rest of the body. Multiply that across about a thousand different units and suddenly you've got an utterly absurd number of nearly identical textures. And since this a strategy game, players are basically never looking at any of them at a level of zoom that affords each unit more than a few dozen pixels.

I think people also have an odd idea of what it takes to photorealistically depict a fucking stadium full of people and players with realtime reflections and sweating normal maps. The texture info alone is gonna be huge, but likewise there are tons of high res models animating in realtime all over the screen. So yeah, this being a gigantic install makes sense if 4k res gaming is what people want to play.

Or uncompressed audio in multiple languages/different soundtrack variants (original/remaster)

Audio is also huge for most games. Especially if the game has a lot of dialogue and offers several languages. AAA, dialogue heavy games can have hundreds of hours of audio for a single language.