I think you are misunderstanding inertial resistant, inertial resistant is the object trying to stay at a constant velocity, when gravity acts on a body of the same mass, they will have same inertial resistant, if an object is made up of more atom or atoms of higher molar mass then the inertial force is higher the reason why they fall with the same acceleration is because gravity does not apply a constant force on the object as a whole, but it act on every matter that made up the object ( proton, nuetron,electron) and gave them force proportional to their mass .And inertia is not related to the weight of the body. And what you see on a scale when you weight the body is not the inertial force, its the normal force which was cause by gravity pulling the body to the surface and the surface is pushing back at the same force. The body that consist of more matter will have more weight because gravity act more on it as a whole.