Why pixels do not count in cameras

I have been seeing manufacturers offer ever-larger pixel counts for newer models. Nikon's D5100 is still in the lineup alongside the newer and less expensive D3200. The D5100 has 16.2 megapixel sensor, the D3200 has a 24.2 megapixel sensor.

Shouldn't that make the D3200 the better camera? But, it doesn't , which serves to point out why you can't shop for a camera based on the number of pixels. It's largely meaningless when it comes to image quality.
To demonstrate why pixels are irrelevant , I took an illustrative image of a corner in a darkened room. Low light brings out "noise" in photographs. It's a kind of grainy unevenness that is generally undesirable although some artists use it for effect. Both photos were taken with the same settings and lens. The grains were more pronounced in the photo from the D3200, which had the higher pixel count.

Why is that? Because at a certain point you're better off having a larger pixel, which captures better quality information about the light it sees, rather than additional pixels capturing lower quality data.

To be clear, I am not singling out Nikon, or either of these cameras. They are terrific products, and I consider the D5100 one of the better bargains in its price range partly because of its ability to capture quality images in low light.

The point here is that when you are shopping for a camera, concentrate on the type and physical dimensions of the sensor. In that case, larger is better because more surface area means more ability to capture light. And though CCD sensors used to be state-of-the-art, CMOS sensors have largely surpassed them.