The plunger head is too light to cause such a motion.
I don't know as I would agree with that. There is a lot of weight when you consider the plunger, the connecting rod and the crank for the plunger. There isn't any counter weights, then you get that spinning pretty good, that is a lot of force.
I do agree that you don't need that much HP to run a baler, but you do need some weight to keep it from rocking around because of the plunger movement.