How Can An IRA Be Worth Tens of Millions?

See this excellent Bloomberg article accounting for Mitt's magical IRA:

The most mysterious of the unexplained mysteries about Mitt Romney’s considerable wealth is how he was able to amass between $21 million and $102 million in his individual retirement account during the 15 years he was at Bain Capital LLC.

How did he do it, given the relatively small amounts that the law permits to be contributed to such a plan on an annual basis?