An amazingly lucid graph in the Economist explains why the middle class has surged in China and is about to surge in India:
At a certain stage it [the middle-class share of the population] starts to boom. That stage was reached in China some time between 1990 and 2005, during which period the middle-class share of the population soared from 15% to 62%. It is just being reached in India now. In 2005, says the reputable National Council for Applied Economic Research, the middle-class share of the population was only about 5%. By 2015, it forecasts, it will have risen to 20%; by 2025, to over 40%.
And in case you were wondering, the article quotes an expert defining a middle class as someone that has at least 33% of income left over for discretionary spending i.e. after paying for food and shelter.