The gravitational force equation actually takes into account the mass of both objects and their distance. The only reason we can throw out a gravitational constant of 9.81m/s^2 for most objects on Earth is because the mass of the Earth is so large that the mass and/or distance of the other object would have to be equally large in order to make any significant change in that value. Technically though, a bowling ball at sea level falls slightly faster than a bowling ball at the top of Everest, as does a bowling ball and feather from the same height. The reason is more accurately that they are experiencing slightly higher or lower gravitational forces, which cause them to accelerate (in conjunction with all the other forces acting on them).