[Gravityloss]

It would make sense to always process the higher resolution image with denoising and demosaicing algorithms and then save that with a much lower resolution as there is less and less information the lower photon counts you use per pixel.

Otherwise you're just saving noise.

Probably if you did a study from people's online photo albums where they just upload originals without thinking, you'd discover that something like 90% of space is wasted on pure noise. Things like the sky or a smooth constant color area that can not be compressed by the jpg algorithm since it is filled with high frequency noise. (Often of the color variety no less.)

There are limits to sensor efficiency. Photon noise statistics gets worse with smaller absolute aperture lenses (focal length times f-number) and smaller field of view (zoom).