Pentax 645Z has a Medium format sensor with dimensions of 44 x 33 mm and sensor area of 1452.00mm2.
Pentax 645Z's sensor is 220% 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 Pentax 645Z Review