[font color=\'#000000\']the small sensors suffer more from noise than the larger sensors, smaller photosites, therefore less signal, therefore more amplification, therefore more noise.[/font]
[font color=\'#000000\']I have to quible slightly: a small sensor only has worse noise if you operate them with the same aperture ratio and shutter speed as a larger sensor. This is an intuitive, common but unfair comparison, since the small sensor is then also giving more DOF (in the normal case that the focal length is smaller in order to get the same FOV): this spare DOF can often be traded away to get increased light gathering ability by using a lower aperture ratio.
Specifically, if instead you use the same aperture diameter and shutter speed with the smaller sensor (so a lower aperture ratio in proportion to the shorter focal length used) the sensor will gather the same amount of light, so have the same signal and need the same amount of amplification, but the smaller sensor size will produce somewhat less thermal noise, so S/N will be the same or slightly better.
The side effect is that a lower sensor ISO setting is used. This approach fails partially if the required lower ISO rating is below the sensor's minimum; but in that case, using the sensor's minimum ISO rating seems to give adequately low noise levels with even quite small sensors and pixels, and this can be compensated for with some combination of a smaller aperture and/or higher shutter speed, and hence more DOF and/or decreased blurring from subject and camera motion compared to the larger format.[/font]