The answers are different because the quantities in question, inertia and volume are not the same thing. Mass and volume are related, but definitely not the same. Let's use some simple numbers to see how this works. Zero acceleration means zero net force. However, all of these forces must add up to zero.
Air resistance depends primarily on the size and speed of a falling object.
For example, two sheets of paper havce the same mass. Drop one sheet and watch it flutter slowly to the ground. Crumple the second sheet into a tight ball, and watch it fall much more quickly. The flat sheet falls more slowly because its greater surface area encounters many more air molecules on the way down, creating a greater resisting force. A gun's greater mass keeps its recoil small.