The flywheels of the old Bullet are approximately 22 pounds.
The comments by others above all touch on various aspects of why the heavy flywheel is advantageous in our engine.
Lightening the flywheel does help to improve acceleration and deceleration.
However, what is almost never discussed is that the rotation of the crankshaft is not anywhere remotely resembling a "smooth rotation". The crank actually rotates in a series of "hitches" that speeds it up and slows it down as it goes around.
It gets a big push of speed on the power stroke, and then it gets slowed down with some resistance by the pumping loss of pushing the exhaust out the valve, and then it sees resistance which slows it as it draws intake mixture in, and then it sees an even harder resistance as it compresses the mixture, and then it gets whipped back into a high speed push as the power stroke delivers to it again.
To say nothing of the effects of reciprocation motion that are happening to it.
The crankshaft actually operates more reliably and better in function when it's heavier.
A single has no help from other cylinders to keep it rotating, nor to help smooth out the rotation. It needs the weight.
Racers do reduce the weight of the flywheels for acceleration and deceleration, and it's good for that, but not good for street use.