One day when we fly in our spacecraft to an alien planet and meet with their leader, I have always naturally assumed that there would be one, i.e. a single government representing the planet. So, an alien would perhaps be surprised when visiting Earth that there are more than 190 countries. 

I wonder if perhaps it is inevitable that we will end up with a single authority, or if there is a natural reversion to a fragmented world, perhaps in the same way that large conglomerated companies become difficult to manage and often end up in decline or split up. 

e.g can we draw conclusions about the benefits of smaller countries or larger countries generally, e.g. political stability, health, growth, crime