If you drop a styrofoam ball and the same size rock ball in a vacuum they will fall exactly the same. It's because they accelerate with the same gravitational acceleration.
While falling both transform their potential energies into kinetic energies, so:
Mass x Grav_accel x Height = 1/2 x Mass x Velocity^2
We can see it does not matter how much weight the object has, because the Mass is on both sides of equation. The Velocity is only proportional to Height so both objects fall the same.
Now if you drop them in air environment - both objects will have to overcome air drag.
The air drag is not dependent on the Mass of object but only on it's shape, velocity, and the environment. If both objects would fall the same, they would both need the same energy to overcome the air drag. This energy is taken from the kinetic energy of the object to push the air molecules out of the way.
But because the heavier object has bigger potential energy from the start (and bigger kinetic energy in the end) the air drag takes relatively smaller part away from the kinetic energy.
Mass x Grav_accel x Height = 1/2 x Mass x Velocity^2 + 1/2 x Velocity^2 x Some_constant
This is why the heavier object falls faster in drag environment.
Now if the objects have same density and one is bigger heavier and the other is smaller and lighter:
Air drag depends on the drag_coefficient which largely depends on the Cross section. Mass (when the density is constant) depends on the Volume.
Volume of sphere is: 4/3 x π x r^3, Cross section of sphere is π x r^2
This means the Mass increases 1.33^(3/2)33 x radius times faster than Cross section for bigger objects, giving them falling advantage.
Thats why dust of the same material falls very very slowly and chunks of the same material fall fast.