Mass does not affect the acceleration due to gravity in any measurable way. The two quantities are independent of one another. Light objects accelerate more slowly than heavy objects only when forces other than gravity are also at work. When this happens, an object may be falling, but it is not in free fall.
Mass does matter when pointed in a downward angle, like if you have a metal toy car racing down a large peice of wood at a downward angle. If you had a plasitic car aong side it to measure which would go faster the metal car will reach the bottom quicker, gravity weighs down things :)
