The One Half of the Coin Flip

God or appropriately powerful being flips a coin to determine which type of world to create. What is the probability that you are in world A versus world B? With no other information beyond these are the two possibilities and you know the fact that you have been born. So a halfer would say one half. But I think a thirder might say, well, we're much more likely to be on world B because world B has a much greater population.