5 ways to build a Romney-sized IRA

Avoid Diversifying

Another strategy: Using a self-directed IRA to make a big bet on a closely held company, said John Olivieri, a partner in the private clients group at White & Case LLP. If the business succeeds, the IRA's value can increase dramatically, he said.

"Those clients who don't diversify an IRA, but instead invest the whole thing in one stock or venture that does really well, will have very high balances," Olivieri said. "Of course, they could also lose everything."

The last strategy is working for a successful company and socking away plenty in your 401(k). Sometimes the tried-and-true way to retirement security works.

Employees of a company with a generous 401(k) program can end up with millions in their account over a career and then roll it into an IRA. Company stock appreciation, annual matching contributions from the employer and additional profit sharing can all add up over a career.

"You can expect some people to have multimillion-dollar IRAs if they include funds from rollovers," Parish said.

IRS Rules

Investors should beware, though, as manipulating IRA holdings for tax purposes can attract attention from the Internal Revenue Service. The IRS prohibits some transactions in accounts such as buying property for personal use with funds or borrowing money from it.

The IRS in 2013 also proposed new reporting requirements for hard-to-value assets in IRAs that were optional this year.

Other curbs are possible. In 2013, President Barack Obama proposed new limits on tax-advantaged retirement accounts, including IRAs and 401(k)-style plans. Under the plan, which hasn't advanced in Congress, people wouldn't be able to add tax- favored contributions once their combined account balances reach about $3.2 million.

According to the Treasury Department, Obama's plan would raise $28.4 billion in revenue over the next decade.

In addition to this year's maximum annual $5,500 contribution to a standard or Roth IRA, those age 50 or older can contribute another $1,000.

401(k) Limits

Contribution limits for 401(k) accounts are higher than for IRAs. Workers can direct as much as $17,500 of pretax income toward their 401(k) in 2014. Employees 50 and older can set aside an additional $5,500.

Most people, of course, have far less than millions in their retirement accounts. The GAO report, based on taxpayer data through 2011, said someone who contributed the maximum amount each year to an IRA from 1975 to 2011 and invested it in the Standard & Poor's 500 Index would have about $350,000.

The total amount could be as much as $4 million per person, though, for those who rolled over 401(k) accounts or used the higher IRA contribution limits for self-employed taxpayers. A further report is expected later this year.

The average balance in an IRA was $92,600 as of June, according to Fidelity Investments, the largest provider of accounts. For workers who have participated in a 401(k) plan for 10 consecutive years, the average balance is $246,200, Boston- based Fidelity's data show.

Among Fidelity customers, 89,020 have IRA accounts totaling more than $1 million and 68,970 have 401(k) accounts with balances larger than that amount, said spokesman Michael Shamrell.