It's basic math and physics.

I will run thru it for you.

For a master cylinder you apply a force to the piston which tries to compress the brake fluid. The pressure increase in the brake fluid is determined by the surface area of the piston and the force you apply to it.

The formula for determining the area of the piston is 3.1415 time the radius squared.

For a 1 inch bore master cylinder the area is 0.78 square inches.

For a 15/16 inch bore master cylinder the area is 0.69 square inches.

How does the smaller bore makes more pressure? You are applying the same force to both. To start with let us assume there is a third bore size that equals 1 square inch of area. Apply 100 lbs of force to that you get 100lbs per square inch. Since the two examples are smaller than 1 square inch you have to calculate how much pressure is being created by understanding that the force applied to each example needs to be corrected to account for the smaller bore size in relationship to one square inch. The formula for that is to divide 1 by the area of the bore you are using.

1 in bore is 1/0.78 = 1.27

15/16 inch bore is 1/0.69 = 1.44

These are the ratios to correct the smaller bore sizes so that they would match a 1 square inch area. We do this so that the end value (pressure out of the master cylinder) can be read as pounds per square inch.

Now let's apply a force to these two sizes, we will use 100 lbs of force, your foot, to make the math easy.

The formula at this point is 100 (force from your foot) times the ratios we calculated.

1 in bore is 100 * 1.27 = 127 lbs per square inch.

15/16 inch bore is 100 * 1.44 = 144 lbs per square inch.

Now the caliper end of things works similarly, but in the opposite direction.

You have a 2.75" caliper piston bore we will need to calculate the area, same formula as above for area, 3.1415 time the radius squared.

Radius of 2.75 is 1.375, square it then times 3.1514 equals 5.94 square inches of piston area.

If you apply the 127 PSI from the 1" bore master cylinder to the caliper piston you get 754 psi on the caliper piston. (formula is pressure into the caliper times caliper piston area)

If you apply the 144 PSI from the 15/16" bore master cylinder to the caliper piston you get 855 psi on the caliper piston.

It's all a case of hydraulic leverage. We will not be getting into the mechanical leverage imparted by the brake pedal pivot points.

An addendum to the above regarding multiple piston calipers. You have to calculate the area of all the pistons involved (some setups use staggered piston sizing) then add those together to come up with the total caliper piston area then you can do the pressure times area math.

For example, some Mustang calipers use two 1.5" pistons. In this case it would be ((3.1415 times the radius squared) times two) so 1.76" area for each piston times two pistons equals 3.54 square inches total. This is much smaller than the 5.94 square inches of the Mopar 2.75" bore caliper.

So pressure on the pistons will be less. But that application (96 Stang) uses a 13" rotor so force at the wheel may be comparable, not doing that math, lol.

Thanks, guys. I'm just very curious and the boundaries of a non face-to-face meeting make it hard to exchange ideas sometimes.
I do understand the math of it. I've read the contributions Feets provided as well. I'm getting the impression that the leverage of the larger sized rotors is where the majority of the improvements are. The thicker rotor width provides a longer duration before they overheat. Newer cars have greater development of pad compounds available too from what I gather.
Since this is a classic car forum, I figure that most of us are modifying old cars one end at a time. Years back, I pulled the stock and perfectly functioning 10" rear drums to install the Dr Diff 10.7" rear disc kit. This was done purely for appearance reasons. I wanted the look of a disc rotor through the wheel spokes. It turns out it didn't make any difference in how the car stopped. Braking performance was the same. That wasn't a bad thing....the brakes weren't terrible before or after. I went from what was probably a 15/16" wheel cylinder to a 1.5" single piston caliper. This is surely apples to oranges but I recall reading that when one had rear wheel lockup with drum brakes, the trick is to use smaller wheel cylinders. Well, a larger sized caliper piston would seem to be expected to provide more clamping force there too.

If you were closer I'd just have you drive my Coronet so you could see what you think. I can tell you that my Coronet stops better than my Toyota 4Runner which has factory power 4 wheel disc brakes. I can exit the freeway at 80 mph in the Coronet and it will immediately pull down to 35 or 25 as I approach stopped traffic. With the 4Runner I have to be careful to leave plenty of stopping distance when slowing down from anything over 70 mph.

My Coronet has 13 inch front rotors with Porsche calipers. I added a rear disc kit a few years back but I think the factory drum brakes that I had on there before worked better than the disc brakes. I had the big station wagon drums out back and they really worked.

That is what I want....brake confidence like I have felt in newer cars. The 72 Duster stops better than any classic car I've owned and it is a basic manual MC 11" disc 10" drum setup with 2.75" calipers. That car is what gives me hope that I can make the Charger stop as well or better if the right selection of parts is found.
Dr Diff suggests his 13" Cobra front kit and the 11.7" rear along with either a 15/16" manual MC or a hydroboost with a 1 1/8" MC.

If you can lock your brakes up you already have more brakes than tire.

You can update your brakes all day long, but unless you have the tires to use that added braking power you are wasting time. So unless you need the added thermal mass of larger brakes look to tire size increases first.

If you already have fitting the biggest tire you can then yes it makes sense to update the brakes, assuming you can no longer lock them up, lol.

Tire size is not a problem.

I don't recall the last time that I was able to or actually did skid the brakes. Right now the engine is out for a rebuild but before I tore it down, the brakes worked okay but weren't impressive enough to brag about.

I think the newer cars can use smaller calipers because of the larger rotors.
Can't underestimate the leverage adder, plus as mentioned, I'm sure the new pads are much better than the old stuff.

Just think of how easy it is to break a bolt free after you slide a pipe on the rachet to extend it.....

and speaking of pads, how do the ceramic types compare to the semi-metallic of back in the day, as compared to "red stuff", "yellow stuff", etc that EBC offers ?
this is in relation to the small rotor size needed to fit 15" wheels.
i have read all the descriptive attributes of the above mentioned pads, as well as what is offered by Hawk and other race type pad manufacturers, but i am hesitant to pull the plug on a set of pads, only to find out they may or may not, work to my expectations.

Excellent point. Could it be that simple?

ABS probably has an effect as well. While I can no longer remember the details I do recall an article Eberg wrote comparing the brakes on his Green Brick vs sports cars of the time it was written. Can't seem to google it though. I don;t recall it being real fancy though it might have has one of AndyF's Viper caliper setups on it.

How much does that Duster weigh?
How much does your Charger weigh?
Bigger cars need more brakes(bigger contact area) to stop as good as lighter smaller cars do, basic physics

This pad part of the thread seems to be getting lost and may need a thread of its own...

good point.

The brake pad topic is a valid one and supports the inclination to change to a modern system. The Dr Diff stuff is based on OEM production calipers and rotors. This likely means that the brake pads might be available from the higher end aftermarket suppliers.