Leica S2 has a Medium format sensor with dimensions of 45 x 30 mm and sensor area of 1350.00mm2.
Leica S2's sensor is 194% larger than the average DSLR sensor in its class. In cases where all the other parameters are the same between two cameras, the one with the larger sensor collects more light per pixel and performs better (less noise) in low light / High ISO conditions . Larger sensors also give the photographer more control on the depth of field.
Read our detailed Leica S2 Review