New Edition
Previously, 90 to 80 percent hollowness was assumed because we didn't know how hollow Necroworld was. But now we have a proper answer. In Transformers: More Than Meets The Eye #50, the RodPod is teleported to the center of Necroworld, and
Megatron states that the distance between them and the planet's inner surface is 3000 miles, making Necroworld's hollow innards to be 6000 miles in diameter, or 9656.064 km.
5.00846009e27 cubic centimeters for the hollowness above.
5.00846009e27 cubic centimeters x 7.85g/cm³ = 3.93164117e25 kilograms (3 * 6.6743015e-11 * 3.93164117e25 ^2) / (5 * 10936883.35) = 5.65993234e33 J 5.65993234e33 J / 2 = 2.82996617e33 = 676.3781477055526 zettatons (5-B, Planet level)
Dayum