Because of the problem of changing one currency to the other or to the US dollars and because of the large disparity in the cost of living between the nations of the world, then fluctuations in exchange rates, it becomes very hard to determine the relative GDP size of nations. Two values are used, that is, nominal GDP and GDP by purchasing power parity. The nominal GDP is arrived at by changing the value of the economy to the dollar value using the exchange rate that is prevalent while the GDP by PPP is arrived at by determining what money can actually do inside a nation within its borders. For example in New York, someone with a 2 thousand dollar income per month is not on a good economic standing and might even be homeless, whereas someone like that in Ilesa Osun state is sitting very pretty and will be seen as very economically comfortable. In well organised countries with a social security system, you need more to survive since government takes up to 30 percent of private income as taxes to support the society as a whole, then all property owners have to pay property tax which sets a minimum on rents. Then slave labour is not allowed which makes things more expensive because of cost of labour. Some industrialized nations repair nothing because cost of repair is like the cost of a new product. For instance, bringing up toddlers is more expensive in industrialized nations as it is against the law to employ other people's school age children permanently in your home to take care of your kids as house helps, a practice common in Nigeria and other backward countries. You are either wealthy enough to hire a nanny at minimum wage or more or you put your toddler in a daycare which is expensive (this in itself is a natural population control). Then you have bills like rent, power, water and gas which amount to multiples of what obtains in under developed or backward countries.

In essence, if you compare places like Ilesa, Akure, Osogbo in Nigeria to advanced cities like London, Berlin or New York, it will become apparent that a 1000 dollar per month income in places like Ilesa, Akure, Osogbo is equivalent to more than 6 thousand dollars per month in places like London, New York or Berlin. So by purchasing power parity a nominal GDP of 100 billion dollars in Ilesa may be the equivalent of like 600 billion dollars in those places. It is largely so because people in those places have joined hands together to help each other, so they need to do more, unlike the all man for himself situation we have in third world countries like Nigeria.

The way to even things up is to have uniform threshholds of taxation globally. Let all governments take thesame percentage of income as taxes all over the world as much as possible and let them be responsible for basically thesame responsibilities. Let property tax be as uniform as possible. Let inflation be as uniform as possible and let there be a global policy that governments should depend solely and only on taxes in their local currency. If currency is inflated, government should not take the proceeds but distribute it to the masses via a lottery. This will remove the incentive for government to be continually inflating the currency as a source of revenue. Such a hamper or curb will bring currency stability and make all currencies a reliable store of value as it should be.

With all these in place and with free trade policies in place, the determinant of the GDP size will be factors like number of workers and size of local markets. Within a few years, China will be number one, followed by India, then the US, Indonesia and etc.