well think of it this way. the d90/d300 both have 12.3 megapixels. the d700/d3 have 12.1, so relatively close. the d700/d3 have less pixels spread over a larger sensor area. this allows the sensor to run at higher iso's while having a higher quality. thats pretty much the simplest way to explain it, im sure there's more to it than just that.

