A = projected area, which is the same if our objects are the "same shape". As for the rocks, you're applying benefit of hindsight. Say I show you a balloon fil

A = projected area, which is the same if our objects are the "same shape". As for the rocks, you're applying benefit of hindsight. Say I show you a balloon filled with air and a balloon filled with water falling at obviously different speeds. Now, you have to explain that with this odd theory of "everything falls at the same speed" -- not the other way around. In the real world, it is evidently true that heavier objects fall slower, and any better theory that disagrees must explain this (e.g., posit the existence of air resistance) before its new predictions matter.