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.