Maybe because the source is often connected to Vdd or GND.
The charge in this capacitors will be constant and a better ripple filtering will be made.
If the drain is more capacitive the circuit will run slowly.
if you take simple case of inverter...out put is at drain.. more drain area means more diffusion capacitance... this capacitance will take some time to charge and discharging resulting in delay in output ..... Hence drain capa is reduced...