Sony Alpha A9 has a Full frame sensor with dimensions of 35.6 x 23.8 mm and sensor area of 847.28mm2.
Sony A9's sensor is 138% larger 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 Sony A9 Review