.kkrieger did that. Not a “real game”, it’s a demo (of a demoscene), a “proof of concept” or a “proof of skill”. Nostalgia Nerd has a very interesting video about it on youtube.
3D models tend to occupy less disk space than textures, as these usually come in at least 2 files: one for the actual colors, one or two more for light mapping (bump map, emission, normals, etc). I don’t know which format NMS uses, but a .obj 3D model with 62k triangles will take around 4.5MB of disk space.
For comparison, this Damaged Helmet in gltf format (which you can see on your browser here) has 15k triangles, a .bin file (the actual 3D geometry) of 545kb and roughly 3MB of textures - The Default_albedo.jpg is the “actual color” and it alone is larger than the .bin + .gltf, at 914kb.
That’s the same problem again, you need hard drive space for all that 3D variation.
Not really. Again, they just need to be smart with what they have. Grassy planet where one third is green grass, another is red grass, another is yellow. No need for any extra stuff to be made, they already have the building blocks. Better yet, mostly grassy planet with patches of radioactive terrain surrounded by desert.
For buildings, just think about player made bases. You can make effectively “infinite” interiors and exteriors with all the stuff players can use to make a base. Write coordinates of “premade” rooms, write some extra lines of code to join specific rooms together and bam, all you needed was less than 10kb of extra text to increase variety.