The incredible shrinking Wall Street banks?

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Like a dog with some really big fleas, the largest U.S. banks are likely to start shaking off some assets.

After decades of swallowing up their smaller competitors, regulatory and political pressure is putting warehouse financial institutions on the firing line to divest their businesses.

While too-big-to-fail banks have only gotten bigger since the financial crisis, analysts view that trend as about to change.

"We expect more divestiture to come from universal banks as higher capital requirements, the Volcker rule and higher liquidity requirements are fully phased in," Keefe, Bruyette & Woods said in a report for clients. "We believe this trend will create opportunities for small and midsized firms in the U.S. and abroad."

Five of the nation's biggest banks control a record 45 percent of the industry's total assets while the eight so-called universal banks hold about 60 percent of the total, according to KBW and SNL Financial.

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However, much of the growth has come in the form of cash on hand as regulations require more of a cushion for banks, particularly those with more than $50 billion in assets. Dodd-Frank rules as well as Basel III guidelines are redefining the way banks will operate in the future.

"Prior to the financial crisis, consolidation was a consistent profitable investment theme in financial stocks," KBW wrote. "Since then, and with the labeling of the country's largest banks as globally systemic, the reverse has been true: the fragmentation of the largest financial firms is creating arguably the best opportunities in the sector."

Indeed, despite a growing cash pile banks have been shedding assets over the past five years: