If the Department of Justice decides to challenge the acquisition of Discover by Capital One on antitrust grounds, new research from the Consumer Financial Protection Bureau will surely play a prominent role in the complaint.

Why it matters: Even though some 4,000 banks offer credit cards, the top 10 issuers, including both Discover and Capital One, account for more than 80% of loans.

• That concentration seems to have given the biggest lenders the ability to jack up the interest rates they charge on outstanding credit card balances.

The big picture: Consumers with poor credit who have a credit card from a small bank are paying a lower interest rate on their purchases (20.6%, on average) than consumers with excellent credit who carry a card from Discover, Capital One, or one of the other big banks. They're paying 23%, on average.

• Credit card interest rates have risen sharply over the past two years, spiking from 14.6% in February 2022 to 21.5% in November 2023.

By the numbers: An American who runs an average balance of $5,000 will end up paying between $400 and $500 a year in excess interest payments if they go with one of the big banks.

• On top of that, a holder of a credit card issued by a big bank is three times more likely to be paying an annual fee than someone with a card from a small bank.

What's happening: The biggest banks have massive marketing operations and credit-card reward programs, which make it hard for smaller banks to compete even when their interest rates are much more attractive.

The bottom line: Capital One would become the largest credit card issuer in America if this deal goes through, with a 19% market share. (JPMorgan currently leads the ranking, with a 16% share.)

• For consumers, however, it's far from clear that bigger means better.

Go deeper: Capital One to buy Discover for $35.3 billion