"The continuing shift to DC plans means they are becoming the world's most prevalent retirement savings model," Steve Carlson, head of Towers Watson's Americas Investment practice, said in a statement.

"This shift brings a transfer of risk and new tension to the balance between ownership and control, which will test governments and pension industries around the world," he added.

Then there's the proportion of GDP that pension plan assets represent. Globally, that's 84 percent, up from 54 percent in 2008. In the U.S., the ratio of pension assets to GDP increased from 95 percent in 2004 to 127 percent in 2014. The U.S. is also the largest pension market, holding 61 percent of total pension assets.

While other countries have been overcoming the so-called home bias in the equity investments they choose for pension portfolios, that hasn't been as much the case in the U.S. 

The degree of home bias varies from country to country, of course, with Canada and Switzerland taking the 2014 prize at 33 percent and 34 percent, respectively. But in the U.S., that bias stood at 67 percent. 

The 16 largest pension markets examined in the study are Australia, Brazil, Canada, France, Germany, Hong Kong, Ireland, Japan, Malaysia, Mexico, the Netherlands, South Africa, South Korea, Switzerland, the U.K. and the U.S. Those nations account for approximately 85 percent of global pension assets. 

The three largest pension markets are the U.S., the U.K. and Japan, with 61 percent, 9 percent and 8 percent of total pension assets, respectively, Towers Watson said.