Making virtual realities that are larger than earth is not a big deal. Why have Virtual realities if they follow every rule of the earth, like size. Limiting size to earth size would mean, for example, that if we where to one day obtain enough information to make a virtual reality of mars, we couldn't turn it into life sized a virtual reality because it would be bigger than earth.
Minecraft is a virtual world, so why limit imagination and possibilities? I think human kind and society should be more concerned about real implications of real problems. Also, I do not believe Minecraft is the first virtual world to be larger than our planet. For example, I wonder if the Middle Earth of the Lord of the Rings is larger. Either way, I think the size of a virtual world is less than relevant.
Without technology, our world would be crippled. The majority of humankind cannot function without it. That being said, it makes complete sense that a world that has been created by online users is 9 million times larger than the earth. Millions of people across the world have worked together to create a virtual fantasy that is beyond our grasp to comprehend. It follows along with society today: technology.
Virtual is fantasy so there's no rules that limit how big/small etc it should be. Surely the fantasy allows developers to free their minds and make the world look how they wish. Terry Pratchett was a great exponent of using his imagination and not letting wordly rules dictate his own virtual reality so why should we?
I know Minecraft is just a game, but for it to be so large means a lot of people spend a lot of time on this. I love playing games too, but this seems a bit excessive to me. People should spend more time on real issues and less time building virtual realities.