Why does a small diaphragm condenser microphone exhibit more self-noise ("hiss") than a large diaphragm condenser microphone?
Two main reasons:
1) The JFET impedance converter that immediately follows the condenser microphone element is the primary source of the noise. The amount of noise is a direct function of the capacitance connected to the JFET input: the greater the capacitance; the lower the JFET noise. A large diaphragm element has greater capacitance due to a greater surface area.
2) The output level of a condenser microphone element is also a direct function of the diaphragm size. Larger diaphragms provide greater output levels than smaller diaphragms. Therefore, a larger signal is fed into the JFET from a large diaphragm microphone and this provides a better signal to noise ratio at the JFET output.