there was an experiment many years ago where objects were dropped off the Eiffel tower to explore the effect. you are wrong.
the fall is due to gravity (whatever that is)
the gravitational attraction acts on every particle of the two balls at the same rate of influence.
so it doesn't matter whether you have a ball full of nothing or a ball full of concrete, every particle of both balls is under the same influence.
you don't get any more influence exerted on the concrete ball.