Much in the same way the wealthiest 1 percent of Americans hold a disproportionate share of the country's riches, the top 1 percent of 401(k) plans owns a lopsided proportion of defined contribution retirement assets.

Of the 540,000 active 401(k) plans as of the end of last year, 5,400, or 1 percent of sponsoring employers, held $3.06 trillion, or 71 percent of all assets, according to an analysis from Judy Diamond Associates, a provider of data and analytics tools for retirement advisors and plan sponsors. 

That the largest employers are dominant is not surprising, given that they employ the majority of Americans enrolled in retirement plans. The largest employers claim 45 million participants, or 56 percent of all enrollees. The balance, meaning that 99 percent, have 35 million participants, or 44 percent of all enrollees in defined contribution plans.