It's not so much to do with how many pixels, but how many there are in relation to the size of the sensor. You can have more pixels on a MF camera than on your DSLR , but the actual size of each pixel can till be larger on the blad - which is good as far as noise is concerned.
When you increase ISO on a digital camera, you're not actually increasing the sensitivity of the sensor, but that effect is achieved by increasing the voltage to the sensor with each increase in ISO.
As the voltage increases, the noise increases too, pretty well proportionately to how nany pixels there are on the sensor, their individual size, and how closely they're crammed together. Lots of small pixels on a small sensor equates to more noise. Improvements are being made all the time, but that's the bones of it.
