Even though volume grows with 3rd power of size and area with 2nd, you'd still better calculate volume here as 2nd power. The reason is that you usually want to

Even though volume grows with 3rd power of size and area with 2nd, you'd still better calculate volume here as 2nd power. The reason is that you usually want to have progressively thicker walls when the size of the object grows, and that - linear with diameter - growth of thickness reduces available weight, so the volume still grows as 2nd power. But even then FH would deliver huge volumes on orbit.