From The New Republic:
While the U.S. financial system has a long tradition of functioning well with a relatively large number of banks and other intermediaries, in recent years, it has been transformed into a highly concentrated system for key products. The big four have half of the market for mortgages and two-thirds of the market for credit cards. Five banks have over 95 percent of the market for over-the-counter derivatives. Three U.S. banks have over 40 percent of the global market for stock underwriting. This degree of market power brings with it not just antitrust concerns, which this administration has declined to act on, and a huge amount of economic risk—but great political influence as well.
Source: The New Republic