unless drum brakes are different because of how the shoes grab the drum and possibly 'self energize" meaning they naturally increase braking pressures by the way the shoes rock back and forth on the backing plate...you won't get more braking force, but your shoes will last much longer because they'll wear much less.
Like I say though, this may be wrong since I know the shoes will rock back and forth on the backing plate and possibly have a natural tendancy to be pulled outwards increasing brake "pressures"
but on disc brakes, if you keep the master cylinder the same, and you increase the pad size, you don't get any harder braking efforts, all that happens is the pad wear goes way down, and heat is spread out over a larger area which will help keep them cooler, longer during hard braking...and in order to increase brake effort, you need to increase brake pressures...thus a larger bore caliper or smaller bore master. when you increase the caliper size, you COULD keep the pad the same size as well, you'll just have much higher pad wear rates.
now, another possibility as to why the rear cylinders are the same, is because of the "torque multiplication" you press on the brake pedal, you make a pressure--psi, that pushes out on the wheel cylinder, pushing out on the shoe...that translates into a certain amount of pounds of pressure. for simplification reasons, let's say you're making 100 lbs of pressure against the drum. at a 10" drum, that's a 5" radius from the pivot point, or, 500 inch pounds of torque acting to slow you down. move that out to 11" and now you have a 5.5" radius, increasing your torque to 550 inch pounds. because again, with disc brakes, the further out your braking force is (larger diameter rotors) the more effective your braking pressures will be.
now, with real numbers...most drum brakes need (I THINK!!!) around 900 psi for maximum stopping power, so that's 900 psi, multiplied by the combined surface area of the wheel cylinders--since they push out in 2 directions, a 1" bore manual has the surface area of (pi x R^squared) x 2. I have no idea what the wheel cylinder bores are...I'm assuming they're around 1" -- 3.14159 x .5^2 or, 1.57 square inches total, times the 900psi, 1413 pounds of force.
1413 lbs x 5" (radius of 10" drums) is 7068 inch pounds...or 589 ft lbs of torque stopping the car...at one wheel.
move that force out by 1/2" by switching to 11" drums, and your 1413 lbs becomes 7771 inch pounds, or 647 ft lbs of torque...without changing anything else.