Retinal neuroscientist Bryan Jones compares the iPhone 5 and 4S displays under a stereomicroscope:
It turns out that the pixels in the iPhone 5 are precisely the
same size as the iPhone 4 pixels, but the iPhone 5 pixels have
better color saturation with more contrast, seen particularly in
the blue pixels. I did not calculate the difference in color
saturation between the two iPhones, but it is pretty clear to the
eye which is which. Apple claims 44% increase in color saturation
and from these images, I believe them.