Increasing the number of pixels while keeping sensor size constant decreases the photon count per pixel because the same amount of light is distributed over more pixels.
SNR: SNR for a pixel is proportional to the square root of the number of photons it captures, as Poisson shot noise dictates that noise level is the square root of the signal (photon count)
Increasing the number of pixels while keeping sensor size constant decreases the photon count per pixel because the same amount of light is distributed over more pixels. SNR: SNR for a pixel is proportional to the square root of the number of photons it captures, as Poisson shot noise dictates that noise level is the square root of the signal (photon count)