There is no extra data to increase resolution, and even the so called '4K assets' are still not all that sharp when you get up close at 1080p. RAM is the limiting factor, hence you have those so called '4K assets' for Xbox One X. That's a limiting factor of the hardware, not the output. They could add options to leave different languages out, sound takes up a lot of space.
However for all these 'new' 120 fps modes, you could leave out a lot of data since those modes don't only need lower output res, but also lower res textures and other assets to keep up.