I dont understand the 20-30% larger surface area from changing the brake cylinder. All the cylinder does is push the drum breaks against the drum with a little rod. The setup of the drum assembly does the rest of the work. Now if it has more pushing force to cause a harder clamp then it could help. But its not like a caliper where larger calipers can take bigger pads to get more surface to clamp. Larger rear cylinders wont change the pad surface.
the pads don't have the larger surface area, the cylinder does. Meaning it can exert more force with the same input and ultimately more braking power. Think about it this way: Frictional force (braking power) = normal force * coefficient of friction. If you increase the area of the pad you increase the frictional force, and therefore increase the braking force. If you increase the area of the piston, you increase the normal force and get more braking with the same friction coefficient.