That announcement was called into question on French radio today. The economist who spoke simply said it was the "biggest" because of the massive population, but the number of extremely wealthy people and those living a sort of middle class life were still greatly outnumbered by those living in poverty.
Statistics can be pulled in any direction that you want. If poverty is the deciding factor in the ranking of countries, the United States was probably not ever #1 at any time since it has always had a lot of people living in poverty.
I believe they are just talking about the GNP. If China can buy the rest of the world, it makes absolutely no difference how many poor people it has.