Many of you that know me will have heard me mention a thing called diffraction when talking about mega pixels and small apetures.
This is all to do with how light rays pass through a small aperture.
For an ideal circular aperture, the 2-D diffraction pattern is called an "airy disk," after its discoverer George Airy. The width of the airy disk is used to define the theoretical maximum resolution for an optical system (defined as the diameter of the first dark circle).
If the airy disk's central peak becomes large relative to the pixel size in the camera, it starts to have a visual impact on the image. This starts to occur once these airy disk's reach a diameter of about 2-3 pixels.
Diffraction appears as a slight softening of any fine details in an image.
This alone is the main reason for me choosing a 16mp camera when I replaced my D700 (12mp) as most of my macro shots are taken at f22