According to the report, it only took two years—from Q3 of 2017—for Fidelity's 401(k) millionaires to add to their numbers by 50 percent—up from 133,000 to their present level.

"401(k) millionaires are different than the average saver," Meghan Murphy, a vice president of thought leadership at Fidelity Investments and an expert in 401(k) plans, is quoted saying in the report.

Murphy adds, "They save at a higher rate— about 23 percent (of annual pay), including employer contributions. They tend to have a bit higher equity exposure. Also, they're extremely engaged. They save consistently. They're older. And they do not take loans and withdrawals from their accounts."

They also make relatively large contributions, Murphy adds, that have managed to offset a lot of negative market action.

So if 401(k) millionaires are adding to their numbers, IRA millionaires aren't standing still. They too have multiplied. While in Q4 2018 their numbers shrank to 138,800, down from 170,400, at the end of Q3 2019, they went the other way, hitting 182,400—up from 179,700 at the close of Q2 and up from 168,100 at the end of Q1.

Millionaires' balances helped to buoy Fidelity 401(k) balances overall, with the average balance in all Fidelity 401(k) accounts rising to $105,200 at the end of Q3—reflecting a decrease of less than one percent from the $106,000 average as of the end of Q2. Fidelity account balances had hit their overall all-time quarterly high in Q3 of 2018, at $106,500. Ten years ago, at the end of Q3 2009, that average was only a tad more than half that, at $59,100.

Read more: 

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• 401(k), IRA, pension contribution limits for 2020: IRS
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