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Andy from Norman Camera supplied me with this image of the sensor’s from three Canon cameras.  The 16 Megapixel fullsized 36 x 24 mm sensor from the  $4000 EOS 1D Mark II is on the left. This sensor is the same size as slide film. The 8 Megapixel  20D, in the middle, has a 22.5 x 15 mm sensor.  The sensor on the right is from the Powershot Pro 1, at 8 Megapixels. Both cameras have the same number of pixels, but the 20D spreads them over a much larger area.  The 20D will deliver better images with the same number of pixels.

The most important factor in image quality is not the number of pixels, but the quality and size of the pixel elements.  Bigger elements gather more light.