Pixel density matters more than the number of pixels, or at least it used to. DP Review stopped listing the pixel density about a year ago as "no longer an indication of image quality".
In the last few years the goalposts have moved dramatically, the new Exmor R sensor from Sony and it's derivatives which are being released currently on an almost weekly basis has changed things.
As an example here is an image from my Pentax Q, a camera with a tiny (1/2.3 Inch) sensor with 12.4Mp crammed onto it, results should be terrible especially as the image was taken in low light with an ISO of 800, even then with the aperture a wide f1.9 the shutter speed still had to be 1/5th of a second, but judge for your self
Jpeg strait from camera
http://www.flickr.com/photos/chris-judge/8173222244/in/photostream/
If you go to the trouble of processing yourself from the Raw file, you can get this. No noise software was used, the noise level is strait out of the camera, i.e. not much.
http://www.flickr.com/photos/chris-judge/8173222996/in/photostream/
An image of this quality would have been impossible just a few years ago, even APS sized sensors would have struggled in this lighting.
The problem now is with the pixels being so small you can't use small apertures as the diffraction they cause at this tiny resolution exceeds the resolution of the light itself and image quality visibly gets worse!
There is the promise of even more to come from sensors.
Chris