Ive done a lot of study and thought on why short bows shoot light arrows better than long bows and can answer that one with certainty. The answer is horsepower vs torque. Short bows have high horsepower and long bows have higher torque. Small little 4 cylinder cars that are light weight can go super fast but a truck with an inline 6 can pull large weight uphill.
The answer is in the engine, and specifically has a lot to do with connecting rod length. The longer the connecting rod the more torque and slower the engine will rotate, vs a short stroke engine that will wrap up fast with a blip of the throttle. The shorter rods ( limbs ) allow for higher rotational speeds and more horsepower which a light load ( arrow ) can be accelerated from. The heavier the load the more torque required to motivate it.
Some comparisons can also be made to the draw length, which is why a longer draw will return at a slower rate than a shorter draw, but deliver more torque. An English longbow takes advantage of both situations. A short bow can be made to draw a long distance but the longer the draw the slower it will shoot a light arrow after a certain point. 22 to 23 inch draw seems to be the peak for super light weight arrow speed, and the arrows weight increases, so too should the draw length to gain the extra torque to speed that arrow up.
