From 1935-1990, the biggest three banks in the United States had 10%-15% of all U.S. banking assets. Since 1990, commercial banking has grown dramatically more concentrated, with the three biggest U.S. banks now holding more than 40% of all U.S. banking assets, up four-fold from 1990.
Notably, this correlates strongly with a time period when pay for top executives in the banking sector soared, while pay for the top executives in the "real economy" increased much more slowly.
I've had doubts that mere size was a major problem in commercial banking, which is much more heavily regulated than investment banking. But, I'm coming to have my doubts.