Its not about "brake swept area per pound". It's about where the braking force is applied and how much axle weight is doing the braking.
If you rely on the towing unit to stop the "train" (train, truck+trailer or highway tractor + A or B train), sooner or later it's going to jacknife. The braking force for each trailer needs to come from the trailer itself.
Furthermore, no tow vehicle by itself is going to be able to generate braking forces like a tv and trailer can as a unit.
Case in point, my trailer weighs about 15,000 lbs. I pull it with an 8,000 pound truck. During maximum braking, I'm at the point of skidding tires with 23,000 pounds on them.
If you pulled my trailer with a 20,000 pound Class 8 tractor and relied soley on it's brakes, you'd have only 20,000 + the pin weight of the trailer at the point of skidding. And you'd be stopping 35,000 pounds, not 23,000 pounds. And the unit would jacknife if you applied the brakes hard and the truck and trailer weren't perfectly aligned or traction wasn't good.
In fact, my truck and trailer with properly working brakes on both would outstop the class 8 truck pulling the trailer with no brakes by quite a bit.
