[quantumstate]

Higher pixel counts give less noise assuming the total light collection area is the same. Normally you get a bit less light collection area due to the gaps between pixels with a higher resolution sensor but overall I think higher pixel count generally wins, especially when working at the low iso end. So as long as the cost is reasonable why not stick lots of pixels in.

[ScottBurson]

No, smaller pixels have more noise (on a pixel basis) because the incoming photons are averaged over a smaller area.

Of course, if you have more resolution, you can reduce the resolution in post-processing, and that will also reduce the noise. But then you might as well have used larger pixels to start with.

So I think the right strategy is, figure out how much resolution you need and how large a sensor you can afford, and then make the pixels as large as possible at that resolution. This appears to be what Canon has done.

[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).