The acceleration of the spheres is constant, even when the mass and force change.
The acceleration is defined as the rate of change of velocity of an object with time. The dimensional formula of acceleration is given by [tex]a = LT^{-2}.[/tex]
Here its clear that acceleration of a body doesn't depend on the mass of the body.
But the force is given by the expression F =ma where m is the mass and a is the acceleration. So the dimensional formula of force will be [tex]F = MLT^{-2}.[/tex]
Thus with the change in mass of the body, the body will feel increased force due to acceleration due to gravity.