More than 1,000 Americans have accumulated IRAs worth at least $25 million, far exceeding the average family's $268,300 retirement savings.

Gregory Baszucki, 61, holds at least $68 million in his IRA—possibly much more. He is among hundreds who bought startup stakes when shares were nearly worthless, parked them in retirement accounts, and watched values balloon to eight figures or higher.

IRAs were designed to help working Americans save for old age through tax incentives. But startup founders, hedge-fund managers, and Silicon Valley insiders have supercharged these accounts by accessing investment opportunities unavailable to most. They use this strategy to build giant balances subsidized by U.S. taxpayers, according to a Wall Street Journal analysis of regulatory filings and government data.

The ranks of mega-IRA holders have surged in recent years, fueled by booming stock prices and a startup formation boom.