Given calipers with the same piston size, the ‘power’ comes from the lever, or correctly – the master cylinder. The bore of the master cylinder, how far the piston moves in relation to the lever movement, any ‘cam’ action (like shimano servowave) built into the lever all affect how much fluid is moved and the pressure it can provide, and how quickly the lever acts on the master cylinder.
Hydraulic brakes are pretty simple when looked at in basic terms, brake fluid doesn’t compress, so you move the piston at the lever and the only thing that can happen if the system has no leaks, is that the pistons in the caliper move.
Decrease master cylinder bore and more pressure is exerted but less fluid is moved. So you need more lever travel to then move the same amount of fluid as before, or a higher leverage ratio on the lever…
All very complex and to get the right balance of feel, modulation and power means different piston ratios/bores/leverage ratios etc.
Also, servo assisted brakes will skew the figures as they will initially feel very powerful but that’s just because the pistons are moving out very quickly initially to grab the disc. That’s bicycle servo, not car – they use vacuum servos to reduce the pressure at the pedal.