(Bloomberg News) Two years after President Barack Obama vowed to eliminate the danger of financial institutions becoming "too big to fail," the nation's largest banks are bigger than they were before the credit crisis.

Five banks -- JPMorgan Chase & Co., Bank of America Corp., Citigroup Inc., Wells Fargo & Co., and Goldman Sachs Group Inc. -- held $8.5 trillion in assets at the end of 2011, equal to 56 percent of the U.S. economy, according to the Federal Reserve.

Five years earlier, before the financial crisis, the largest banks' assets amounted to 43 percent of U.S. output. The Big Five today are about twice as large as they were a decade ago relative to the economy, sparking concern that trouble at a major bank would rock the financial system and force the government to step in as it did during the 2008 crunch.

"Market participants believe that nothing has changed, that too-big-to-fail is fully intact," said Gary Stern, former president of the Federal Reserve Bank of Minneapolis.

That specter is eroding faith in Obama's pledge that taxpayer-funded bailouts are a thing of the past. It is also exposing him to criticism from Federal Reserve officials, Republicans and Occupy Wall Street supporters, who see the concentration of bank power as a threat to economic stability.

As weaker firms collapsed or were acquired, a handful of financial giants emerged from the crisis and have thrived. Since then, JPMorgan, Goldman Sachs and Wells Fargo have continued to swell, if less dramatically, thanks to internal growth and acquisitions from European banks shedding assets amid the euro crisis.

'Few Massive Firms'

The industry's evolution defies the president's January 2010 call to "prevent the further consolidation of our financial system." Embracing new limits on banks' trading operations, Obama said then that taxpayers wouldn't be well "served by a financial system that comprises just a few massive firms."

Simon Johnson, a former chief economist of the International Monetary Fund, blames a "lack of leadership at Treasury and the White House" for the failure to fulfill that promise. "It'd be safer to break them up," he said.

The Obama administration rejects the criticism, citing new safeguards to head off further turmoil in the banking system. Treasury Secretary Timothy Geithner says the U.S. "financial system is significantly stronger than it was before the crisis." He credits a flurry of new regulations, including tougher capital and liquidity requirements that limit risk-taking by the biggest banks, authority to take over failing big institutions, and prohibitions on the largest banks acquiring competitors.