It seems to me that the DX mode of 15.3 MP is rather small compared to the huge FX size. DX has been 12 MP for some time now so why do we only gain 3 MP when the FX mode has more than doubled from the D4. Sensor is much larger right ? Or am I way off base with my thinking ?
Bob, DX has always been by definition a crop 1.52x smaller width and height compared to 35mm film, which is very close to the same size as FX. Therefore, the pixel count follows the definition: 36MP/1.52² = 15.3MP.
The state-of-the-art DX camera has been the D7000 at 16MP for almost a couple years now, which is nearly the same pixel size of the D800. The D700 DX crop was just over 5MP, which as you can see means D800 DX crop tripled over D700, just like the FX pixel count tripled.
If you're asking why did FX increase its pixel count 300% in the jump from D700 to D800, while the DX crop is only a 33% increase over the DX cameras from the same era... It's because FX never had the pixel density of DX cameras from the same era. This aspect of DX has kept a lot of shooters from moving to FX, until the D800 came out, which finally puts FX on the same pixel-density ground as the current DX cameras.