Smaller pistons, smaller brake pads - no doubt they are making road discs weaker on purpose.
Braking power is independent of brake pad area. The area of the caliper's piston matters, but not on its own; the hydraulic leverage ratio is determined by the ratio of the areas of the master cylinder piston and the caliper piston. Power is determined by the leverage ratio and the pad's coefficient of friction against the disc. It sounds counterintuitive, but pad area doesn't affect braking power.
why does a smaller brake pad = less stopping power. i was taught in physics that frictional force is independent of surface area.
Ding ding ding! We have a winner! A smaller brake pad doesn't
equal less stopping power. Somewhere, a physics teacher is smiling.
Two brakes with the same leverage ratio and the same pad material will have the same power. It doesn't matter if one caliper piston is 20mm in diameter and the other is 10mm in diameter; as long as the lever pistons are sized proportionally, the brakes will produce the same power.
Hydraulic systems are essentially liquid levers. If a 20mm MTB caliper is driven by a 10mm lever piston (areas in a ratio of 4:1), then the 10mm road caliper would need to be driven by a 5mm lever piston to produce the same power (these areas also have a ratio of 4:1). A 10-pound force on either lever piston would produce a 40-pound force at their respective calipers. The difference is that the fluid would be under considerably more pressure in the road system than the MTB system for a given braking force. The smaller pad would also be generating the same amount of heat with a smaller mass, so it would heat up faster and reach a higher peak temperature than the larger pad.
Let's say these two brakes happen to have pads the same diameter as their caliper piston. The 20mm-diameter pad has four times the surface area (314 mm^2) of the 10mm pad (78.4 mm^2), but they produce the same frictional force. Strange but true.
Essentially, friction has nothing to do with contact area. It's self-canceling: if you double the contact area, you cut the contact pressure in half. So why do race cars and racing motorcycles have giant brakes with six-piston calipers? It's not to get more braking power; tires and their coefficients of friction dictate that. Race cars and motorcycles use big brakes not to stop quickly but to dump heat quickly. Six-piston calipers are mostly about packaging efficiency: they let you spread the heat generation over a large pad area while still managing to fit the caliper and disc between the hub and the car rim.
This is why it's silly to get a big brake kit for your street car: if you're not on a track, you're never using the increased heat-dumping capability those kits bring. Sure, the brakes feel different, but that's due to woven steel hoses, pads with a higher coefficient of friction and generous dollop of the placebo effect.
Some people point out that sports cars generate more grip by using wider tires. It's tempting to jump to the conclusion that the increased contact patch area is what improves adhesion, but it's not true. (Also, wider tires have the same contact patch area as narrow tires unless you lower the pressure). Sports/ race cars use wide tires essentially to manage heat and to improve the shape of the contact patch. A narrow car tire tends to take all the cornering loads on its outside shoulder, and this leads to a failure mode called "chunking" or "cording." Wider tires mean you can distribute heat over a larger area, which staves off tire failure. Race car tires are often heated above the boiling point of water, so heat management is a huge deal. In fact, racers use infrared imaging (and tire pyrometers) to ensure that the tire is heating up evenly across its width. They'll adjust tire pressure to even out the heat contours, avoiding tire failure due to overheated (and chunked) outside edges.
So yeah, mountain bikes don't have more stopping power because their tires are bigger. Once your brake is strong enough to lift the rear wheel--on road or off--max stopping power is dictated by where your center of mass is. Low and far behind the front wheel is what you want here...this is why tandems can stop faster than single bikes.
Smaller discs mean it takes more hand force to produce the same amount of power. That's one reason 140mm rear rotors make a ton of sense for both road bikes and XC bikes (though not for tandems). Time will tell whether 140mm can dissipate enough heat If the leverage ratio is different between two brakes, the power they produce per unit of hand force will be different. You can't just compare caliper diameters between brakes because caliper diameter doesn't matter: caliper piston diameter doesn't matter unless you also know the master cylinder diameter. Even then, you'll need to know the ratio between lever travel and master cylinder displacement.
The upshot is that it's really hard to compare the power of mountain bike brakes vs. road bike brakes unless you know a lot about each brake. The easiest thing to do is just to ride both and compare that way. There's a huge margin of error, but perceived power is what people actually care about anyway.
Physics aside, I've ridden Sram's Rival 1 hydraulic brakes, and they're certainly less powerful than my mountain bike's XTR Race brakes. I too would like the ability to do 1-finger stoppies on the road...I'm curious to try Etap HRD and the new Dura-Ace hydraulic discs to see if they're actually down on power like the SRAM brakes I tried.