It is something anyone can predict. China and India are two very special countries. A country is made up of people and it is only as big as its population. No country even comes close to these two in size. So they will naturally be the two biggest economies in near future. The current situation is due to 1) the resources, like USA was a powerhouse of material resources when its time as a major power began in 1945, and 2) technology, more technologically advanced countries will have larger economies like european countries. But possessing resources will not be important for a country in future. Japan is a good example, it doesnt have material resources but still is a big economy. And more importantly technological differences will vanish in future. It will be impossible to keep know how secret. Its already started to happen. So the bigger economies will simply be the bigger countries. Just like there is more honey in a hive in which more bees live.