the heavier prop should absorb vibrations just due to the increased mass...

you're right about the acceleration, especialy with single-cylinder engines, since there is power applied to the prop once per revolution (or every 2 for a 4 stroke) the combustion accelerates the prop for about 1/4 of a turn, then the prop decelerates the rest of the way round untill the next combustion, so the prop is accelerating and decelerating constantly. a heavier prop will act as a flywheel and smooth out the power pulses, but the larger prop will also put more load on the engine, making acceration harder, and the deceleration more profound, which gives you more engine-movement in the rubber mounts. the rubber mounts serve to smooth the pulses by allowing the engine to move counter to the prop (violently) instead of jerking the prop up to speed, as the rubber de-compresses the engine returns to a normal postion, but since the crank has moved on by then, that power isn't put back into the prop speed, so you effectively lose HP at the prop through the soft mounts. this engine movement is completely different from vibrations due to being out of balance, it is a pure torque reaction. at higher rpms, the engine can't move back and forth completely between each combustion, because of the dampning action of the rubbers, so it apears to vibrate from the power-pulses.
I believe the only way to tell the difference between balance induced vibration and power pulseing would be to remove the plug, and spin the engine with an attached electric motor (a starter would dampen the vibrations).