The dollar soared to an almost-seven-year-high against the yen Monday, following the Bank of Japan (BOJ)'s announcement Friday that it's expanding its easing program.

So it stands to reason that those investors betting on the yen's decline made out big-time. And one of them was Soros Fund Management, chaired by hedge fund legend George Soros, according to The Wall Street Journal.

The firm has earned hundreds of millions of dollars shorting the yen since the BOJ's announcement Friday, a knowledgeable source told the paper.

The dollar traded at 113.63 yen Tuesday morning, up 4 percent from Thursday's close of 109.21 yen.

Other major money-management firms benefiting from the yen's plunge include Discovery Capital Management and hedge fund star Eric Mindich's Eton Park Capital Management, sources tell The Journal.

"It's one of the most widely held, crowded positions in all of hedge funds," Adam Taback, president of alternative strategies at Wells Fargo Private Bank, tells the paper.

"We've been expecting that . . . the BOJ would have to do more, because growth and inflation continue to come in below target," says Scott Mather, chief investment officer of U.S. core strategy at Pimco. "They just acted sooner than most expected."

The dollar also climbed to a two-year high against the euro Monday, as the European Central Bank enlarges its stimulus too.

"The market is likely to be favoring the dollar purely because the alternative economies are more dovish than the U.S. and the Fed," Harry Adams, head of trading at Argentex, a currency advisory company in London, tells Bloomberg.

"Something will have to change significantly for this trade to be altered."

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