the booster doesn't deal with the weight of the car as much as it deals with your foot having to push hydrolic fluid through the lines. The calipers ( 1,2,4 piston), rotor size, brake pad size, are weight/ power related. a heavier car would need bigger of these, as would the light car with a few 100 extra hp. the booster simply transfers the little pedal effort you give into a stronger force to push the hydrolic fluid through the lines, try disconnecting the vacume line from your booster and see how the brakes feel.

simply, it makes it less effort for a driver to stop, not more pressure for the brakes.