There is a combination of weight, rotating mass and gearing that makes it easier for a Triumph to keep up than you would think.
The big twins weigh 60 to 300 pounds more than a Triumph twin and it takes power to move weight.
The HD twin has 2 large flywheels in the engine along with an alternator rotor and a very heavy clutch assembly while the Triumph has a lighter clutch, a similar alternator rotor and no internal flywheel, so the Harley has a lot of metal to wind up and that slows acceleration.
The big twins redline at 5500 and the Sportsters at around 6200 RPM. That means they have to be geared a lot taller to run the same speed as a Triumph. This means that the Triumph has more leverage to pull with, so it will make just as much force with a smaller amount of torque.
From personal experience, the 88 inch twin cam takes a lot of modification to keep up with my America. A 1200L Sportster will give it a run for its money, but will not leave it behind. A modified 96 inch Softail will pull a little better at the top end, but a 96 inch Roadking with the same mods won't keep up.
