>>718178987 (OP)
Every time you increase textures by one level you increase their size by 4-5x if uncompressed. (5x because MipMapping)
Additionally, Skyrim came out in an era BEFORE physically based rendering so they only have 3 textures per model (color, specular and normal maps). PBR typically also adds a metalness map on top.
IIRC Skyrim's textures are by default only 512x512 while Starfield has resolutions ranging from 1024x1024 to 2048x2048 (as well as 2048x1024 or 1024x512)
Skyrim SE by default includes the high res texture pack from the original which bumps up resolutions from 512x512 to 1024x1024. Vanilla Skyrim's textures came to about 2-3GB in total while SE's are about 6-8GB, so that checks about out.
Starfield now adds the aforementioned metalness map on top of everything which increases the texture load of all mashes by 33% (3 textures that we had prior + an extra one). Additionally the total amount of over 100,000 texture files. Skyrim "only" has about 32,000.
So not only is the average texture about 4-8x higher in file size due to the resoluion bump, you now have far more too.
Used to be that audio files used to be the biggest portion of a game's install size (which is why games could only use non-MIDI OSTs and recorded voice once the CD was a thing and they often didn't even get installed with a game but you had to have the CD inside to hear them because that voiced content was sometimes bigger than the entirety of the HDD space)... now Textures have taken that place.
