Olympus PEN E-PL9 has a Four Thirds sensor with dimensions of 17.3 x 13 mm and sensor area of 224.90mm2.
It is 80% smaller than the average Mirrorless 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 Olympus E-PL9 Review