If you judge a nation by its millionaires, the you're going to hold the US in high regard.

But China is actually creating millionaires at a faster clip than US.

The data come via CNBC's Robert Frank, who handles the wealth beat for the financial news network. Citing the "Global Wealth 2015: Winning the Growth Game" from Boston Consulting Group, Frank notes that America has 6.9 million millionaires, while China has a mere 3.6 million.

But China added 2 million millionaires in 2014 alone, a nearly 50% increase from 2013.

This makes sense, of course: China has plenty of economic running room, relative to the US, with its mature economy.

The dark side to this trend, however, which Frank points out. Millionaires are on track to "control nearly half" the world's private wealth by 2016.

That's a lot of wealth: $164 trillion, according to BCG.

And why do the wealthy control so much wealth? It goes back to the argument put forward by French economist Thomas Piketty last year, in his book "Capital in the 21st Century."

Piketty's research, which has been widely debated, suggests that the gains of those who control financial assets is outpacing those who make their money through labor and production, which is represented by economic growth. In developed economies, growth — expressed as GDP — has been anemic during the recovery from the financial crisis. Asset wealth — stocks, bonds, real estate, and so on — has by contrast surged.

Basically, the rich get richer, while wage earners see little or no gains.

It's a good time to be a millionaire, for as long as economic policies worldwide encourage investment over wage earning (Piketty's out-there solution is a global wealth tax).

But there's a wrinkle: we're seeing more millionaires, but the gap between millionaires and billionaires is widening. So even if you're a millionaire, there's reason to think that you're falling behind.