Members with 4 wheel disc systems: Can you lock your wheels?

Not sure that is related the "surface area" so much as it's related to caliper piston diameter front to back. With drums it's wheel cylinder size.

What I'm not understanding is how 12" has more surface are than the 13" setup.

Are the caliper pistons bigger on the 12" setup than the 13" setup?
The 12" caliper had 2.75" pistons, the 13" caliper had two pistons that were 1.58" each (which was less area than the 2.75" caliper)