Let's consider a CMOS sensor with N number of pixels. For the sake of simplicity, let's consider an hypothetical Black and White sensor. Each pixel catches the energy provided by the input luminance seen through entrance pupil of the lens. It generates a charge (Coulomb) per pixel area which is proportional to the input luminance. This charge is read through an external circuitry and eventually amplified. The output signal is a voltage or current which is proportional to such a charge and hence to the input luminance.
So, the image signal for each pixel is a current (or a voltage) proportional to the input luminance. Now, how is such a signal displayed?
More Current provided by a certain pixel --> More white on the screen on that pixel? If it works in that way, the perceived luminance of an image will depend on the pixel area
More Current provided by a certain pixel normalized to the pixel area --> More white on the screen on that pixel? If it works in that way, bigger pixels does not imply brighter images