The force that the larger piston can exert is directly proportional to the difference in the surface area of the two pistons. Assuming that the pressure is the same on both pistons, the larger piston can exert a force that is equal to the force applied on the smaller piston multiplied by the ratio of the surface area of the larger piston to the surface area of the smaller piston.
For example, if the surface area of the larger piston is four times larger than the surface area of the smaller piston, then the larger piston can exert a force that is four times greater than the force applied on the smaller piston.