It is quite common that multi-piston calipers have smaller diameter pistons, this will ensure more uniformity in the pressure distribution on the brake pads.
To add, by having pistons of differing sizes in the caliper, our brake designers are able to tune the distribution of pressure across the brake pad. This can allow them to improve the brake noise, compensate for brake pad wear, make changes in the brake output, or make other performance changes in the brake behavior.