This is due to large senor and lower pixel density

thus the quality per pixsel is lot better than a smaller DX pixel.
and this is the advantage of larger sensor but moderate MP, but if you increase the MP to 24mp, then the pixel quality will drop, it will still be better than D300, due to larger size pixsel, however it will not be as good as the D3, so as far as the D3 is concerned, it is the high ISO performance and pixel quality king, the next crop is not likely to improve pixel quality, but certainly lot more total pixel.