Scale and Composition of Earth’s surface: crust, water and atmosphere
Iron oxide is 5x denser than water, so that's why the spheres are different volumes. All densities are factored in, even the crust plus water sphere is a weighted average of coating plus water density.
Silicon dioxide (SiO₂) has an equivalent size to the entire crust plus water sphere. That's just an illustration of the fact the volume grows as the cube of the radius if you make the radius of a sphere only 26 percent larger the volume doubles! It's also why our planet looks almost a similar size.