StarCraft 2 was released in 2007, and a quick search indicates the most common screen resolution was 1024x768 that year. That feels about right, anyway. A bit under a million pixels to render.

A modern 4K monitor has a bit over eight million pixels, slightly more than ten times as much. So you'd expect the textures and models to be about ten times the size. But modern games don't just have 'colour textures', they're likely to have specular, normal and parallax ones too, so that's another three times. The voice acting isn't likely to be in a single language any more either, so there'll be several copies of all the sound files.

A clean Starcraft 2 install is a bit over 20 GB. 'Biggest' game I have is Baldur's Gate 3, which is about 140 GB, so really just about seven times as big. That's quite good, considering how much game that is!

I do agree with you. I can't think of a single useful feature that's been added to eg. MS Office since Office 97, say, and that version is so tiny and fast compared to the modern abomination. (In fact, in a lot of ways it's worse - has had some functionality removed and not replaced.) And modern AAA games do focus too much on shiny and not enough on gameplay, but the fact that they take a lot more resources is more to do with our computers being expected to do a lot more.