Recall the famous story of Galileo dropping cannon balls from a tower to demonstrate that balls of different sizes fall at the same rate. This showed that gravity pulls more strongly on a big ball than on a small one. Suppose this was not so, and the force acting on both balls was in fact the same. In that case, by Newton’s second law $$a = F/m$$, the smaller ball would accelerate faster. If it were light enough, it could zip to the ground like a bullet! (Conversely, a sufficiently massive ball would seem to hang in mid-air, a prisoner of its own inertia.)