WASHINGTON (MarketWatch) — Mitt Romney isn’t the only millionaire who’s managed to exploit U.S. tax laws to sock away hundreds of millions of dollars in a tax-deferred individual retirement account, the Government Accountability Office reports.

The GAO report shows that the top 1% have saved $1 trillion in their IRAs, 22% of the total.

It’s no surprise that rich people have a large share of wealth, but it is a bit surprising that they own such a large percentage of IRA assets, which were designed to help middle-class people save a few hundred thousand, not to help billionaires save a few hundred million.

“ The IRA was never intended to be a tax shelter for millionaires.” ” — —Sen. Ron Wyden

The IRA is not supposed to be a giveaway to millionaires. But that’s what it’s become.

“Concerns have been raised that tax benefits accrue primarily for higher-income individuals,” the GAO says in its usual monotone.

Democratic Sen. Ron Wyden of Oregon used stronger language at a hearing at the Senate Finance Committee this week.

“Something is out of whack,” Wyden fumed. “The IRA was never intended to be a tax shelter for millionaires.”

While millionaires take advantage of “sweetheart deals” to avoid taxes, the typical American has saved only about $59,000 for retirement, Wyden pointed out. A third of Americans can’t save anything.

Recall that during the 2012 election, Romney released tax documents showing that he had between $20.7 million and $101.6 million in his IRA accounts. It became a minor campaign issue, not just because it put the spotlight on Romney’s wealth but also because it revealed just how easy it is for the wealthy to take advantage of tax loopholes to amass even more wealth.

The GAO report shows that Romney was a piker when it came to avoiding taxes on his millions.

As of 2011, 314 multi-millionaires had more than $25 million saved in their IRA, with average holdings of $258 million, the GAO reported. About 9,000 taxpayers had at least $5 million in their IRA, with average holdings of $16 million.

All told, 630,000 millionaires — about 1% of all IRA savers — cumulatively had more than $1 trillion in IRA accounts, accounting for 22% of all IRA assets.

Meanwhile, the other 99% — the 42 million taxpayers whose IRAs held less than $1 million — had average savings of just under $100,000.

There are two main ways to accumulate assets in an IRA: 1. Contribute up to the maximum each year. 2. Roll over a distribution from a defined-contribution pension — such as a 401(k) — or from a defined-benefit pension plan.

It would be nearly impossible to accumulate $5 million in an IRA using those two methods, the GAO found. If a couple contributed the maximum every year since 1975 (when the IRA was invented), they would have about $350,000 today if they had invested it all aggressively in the S&P 500 Index SPX, -1.11% . A couple who rolled over the maximum from another pension could have earned about $4 million if they invested 100% in stocks.

But only a few people contribute the maximum to an IRA or defined-contribution plan in any year, the GAO says. So it’s extremely unlikely that many people contributed the maximum for 35 years.

If it’s nearly impossible to accumulate $5 million, then how did those 314 taxpayers accumulate an average of $258 million? Perhaps they were very fortunate in their investments, buying Microsoft MSFT, -1.24% , Apple AAPL, -3.17% , Google GOOG, -2.37% , Wal-Mart WMT, -1.02% , Berkshire Hathaway BRK.B, +0.07% and Eaton Vance EV, -0.82% at the bottom and riding them to the top.

Or maybe they took advantage of a trick Romney used to fund his IRA: putting undervalued non-publicly traded assets in his IRA to stay under the maximum contribution limits, and then watching those investments turn into gold.

According to the Wall Street Journal, that’s what Romney and others at Bain Capital were able to do to achieve astronomical returns in their IRAs. Bain took over companies and allowed its employees to invest in those deals. After turning the companies around, Bain sold them, and the employees who invested earned returns averaging 50% to 80% annually, the Journal reported.

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But that wasn’t enough.

“Bain added a couple of unusual twists that made co-investing even more rewarding,” Mark Maremont of the Journal reported. “It allowed employees to co-invest via tax-deferred retirement accounts, and to do so by buying a special share class that cost little but yielded much larger gains than other shares when deals proved successful.”

In essence, Bain would value the special, riskier shares at pennies on the dollar. In one deal, employees invested about $23,000 in their IRAs. When the takeover target went public, those shares were worth about $14 million, and were worth about $23 million they finally sold the shares. That’s a 100,000% return.

Those are the kind of “sweetheart stock deals” that Wyden complained about. They may be legal, but they violate the spirit of the law, which is to limit contributions so that middle-class families can get most of the benefits of the tax breaks.

Taxpayers spend $140 billion a year subsidizing retirement savings, with about $20 billion going to the top 1% of earners. If we’re going to subsidize savings, let’s help those who really need it, not millionaires and billionaires.