Possible Duplicate:
Don’t heavier objects actually fall faster because they exert their own gravity?
When true: The force pulling the "heavy" object down is greater BUT it also takes more force to accelerate a heavy object. These two effects cancel out.
As I heard, the bigger mass/size an object the bigger attraction force between each other, then, why the big mass object did not accelerate faster due to the attraction force is greater than light object.