In the Transformers Prime episode Deadlock, the Deceptions constructed a new Omega Lock with the intention of cyberforming the earth, which we know has Unicron for its core. After the Autobots obtained it, Ratchet told Optimus that it might not work for restarting Cybertron, which has Primus for its core, because the Cybertron is so much larger than earth. Since both planets contain the bodies of assumedly equal beings of creation and destruction, why was Cybertron larger? In One shall Rise, Optimus theorized that the reason no one noticed Unicron within our planet was the fact that debris from space slowly gathered around his slumbering form, gradually forming the planet we live on. This, however, would lead us to the conclusion that earth should be larger than Cybertron, not the other way around.
Why is Cybertron larger than earth? Is there significantly more matter in Cybertron than what was originally a part of Primus? Alternatively, if Unicron was smaller than Primus, can anyone explain why? Was there ever a definitive statement of proportional size, or in-cannon evidence of battles (with the 13 or otherwise) where he lost mass?
Also, please note that this question is about transformers-prime, anyone who points out Gen-1 size inconsistencies without tying them to this continuity will not be appreciated.