McKinsey's definition of middle class－households earning from 60,000 yuan ($9,600) to 229,000 yuan annually－would encompass 45 percent of all households in 2022. "Central to this huge surge in middle-class consumers has been the country's urbanization, and with it, the creation of higher-paying jobs," wrote Gordon Orr, chairman of McKinsey Asia, in a March 2014 post.

Cui, the researcher at GavekalDragonomics, found that while overall wage growth has been strong in recent years, the biggest beneficiaries have not been the middle class.

Instead, the average monthly take of low-income migrant workers has risen by 70 percent over the past five years, much faster than urban wages. That has happened as growth in the supply of new migrants moving to the cities has slowed－in part because of the one-child policy－even as demand for blue-collar workers has remained high.

The rapid expansion of college enrollment in recent years has also boosted the numbers of prospective white-collar workers, putting downward pressure on that group's wages.

While higher incomes for China's migrants have indeed boosted their buying power, it has had a limited impact on overall consumption growth because their total spending is still very small.

The bottom 40 percent of China's households, which include migrant workers, account for only 17.5 percent of total consumption, according to GavekalDragonomics. "That is the obvious consequence of the bottom 40 percent having incomes that are less than half the national average," the report noted.

"The key to China's future consumption growth is thus not how fast the wages of rural migrant workers rise, but how quickly middle-income workers make gains," the report said.