the cylinder has a larger internal bore so it moves more fluid for the same pedal pressure
the larger servo then assists it to a much higher ratio again

imagine 2 syringes, a small one of say 5cc and a larger one of say 10cc, if they are the same length then pushing one in half way will move half/double the volume so you will have not only more fluid acting upon the brake pads, you can use larger pads/more pistons to give you better clamping

it all gets very complicated if you go into pressures and surface areas and all sorts but suffice to say that it is all you will need realisticlly to power the vast majority of calipers out there, regardless of the number of pistons they can go up to

but james does highlight another potential problem, not only will you need to create a vacuum, you will also need to create a vacuum take off from the manifold that's sole job is to provide the assistance for the servo