Why wouldn't you choose larger caliper pistons?

[Olds.PhD]

So, I'm finally looking at bumping up from my 15" wheels to 18", which allows me to get rid of the weak and incredibly vague factory 11" discs on my 68 Cutlass. Originally, I was planning on going with KORE3's Z06 package, but it looks like those calipers are likely to interfere with any wheel I get (18x10 with 6" backspace - apparently doesn't work for Rocket Racing Attacks, and I'm not holding my breath on American Racing's VN514s). But it looks like Wilwood's AERO6 big brake setup would likely work. However, in researching this, the kit calipers come with 4.04 square inches of piston area, but they also sell, in the same physical package, 5.4 and 6.52 square inch versions.

If increased piston area gives you greater clamping force, and the factory discs for a GM A-body are massive single pistons with something like 6.7 square inches of piston area, why would you ever run the smaller piston versions of that caliper? Especially since the caliper itself is the same physical size.

[dontlifttoshift]

Math. Math is why you wouldn't choose the larger pistons.

That said, I do this often. Wilwood will sub calipers of different piston diameters in a kit if I order direct and with manual brakes upsizing the front calipers is a good move. With power brakes that advantages of the larger calipers are less. You can also fix marginal power assist system where the booster is too small or the vacuum is not enough by upsizing calipers or down sizing the master cylinder.

Remember that a 20% increase in piston area is going to need ~a 20% increase in volume. That will either come from pedal travel or master cylinder size. If you upsize the master cylinder you will have less line pressure for a given pedal input pressure and you end up with a net zero gain.

Too much brakes is a thing, too. I see this with hydroboost systems a lot as the system makes sooooo much line pressure. When you pair that with remotely decent brakes you get a system that "you barely have to touch, it's great!" that will absolutely suck if you panic stop.

[Olds.PhD]

I guess that's what's confusing here though; we aren't increasing area - we're decreasing it. The GM A-body already has a massive piston area. The master cylinder is already sized for the larger piston size from the factory. By going with the standard kit you're losing 60% of your piston area while increasing the disc radius 27%. From what I'm finding, using online calculators, that results in a pretty substantial decrease in brake torque (~20%) if the master and input force remain the same. You can get back to even by dropping the master cylinder from 1.125 to 1.00, but then, what are you really gaining? You've still got the same stopping power. You're definitely gaining thermal capacity with the larger rotor. Is that really it though? Does this improve your braking feel? Make modulation easier/harder? Some other benefit I'm not thinking of?