I think I got it. The D600 DX mode crop surface area is more or less the same as the D7000. However the pixel count is less for the D600 and hence the smaller file size. The overall MP of the D600 is larger than the D7000 but it is in the FX mode. When you compare the actual "pixel density", the D7000 is 56% more than the D600. And that is not insignificant. Now with the D7100, the difference is even more, 86%. That's almost double. The answer to my real question is, what to upgrade from my D7000. I don't think the D600 will do if I am to continue to use my DX lenses and end up with lesser quality images in the DX crop mode than my D7000.