My understanding ( and again it is pure terminology ) heavy not because it feels heavy on the raquet. But because it behaves as a heavier object as it really is. Hits the ground harder than it suppose to.
Heavy ball is where downward speed is dominated by the Magnus Effect ( not by only gravity force, called gravity balls ).
Magnus effect needs both speed and spin. Spinny slow balls or fast flat balls are all gravity balls.
Regular ball will never jump off the court higher than the apex of its trajectory. If it is cleared the net at 4ft it will not jump over 4ft off the court.
At some point, when speed and spin are high enough the ball gets so much extra downward speed from the Magnus force that it can jump higher than it cleared the net.
I pretty often play against guys who hit low over the net but if I step back the ball will rise over my shoulder. I play such balls myself and on my opinion this is the most difficult ball to deal with. Even if it goes straight to you you cant immediately predict the way it will bounce and often mistime it. It also moves across you field of vision fast. I would rather prefer getting a faster flat ball.