Small area photodiodes have low capacitance. To maintain low noise operation it is important to keep the input capacitance low so that the miniscule input noise current of the CMOS amplifier, and not the voltage noise, dominates the overall circuit noise. The total input capacitance from the amplifier in this circuit is the differential input capacitance (3.5pF) in parallel with one common mode input capacitance (2.1pf, due to the +input being grounded). The photodiode capacitance is only 1.8pF so the amplifiers C dominates.