Thousands of people flooded into credit unions and small banks over the weekend as part of Bank Transfer Day, an effort to prod depositors to abandon giant banks. But at least some of the big banks won't mind losing those customers.

On Saturday, the Boeing Employees' Credit Union in Seattle signed up a one-day record 659 new members. At the grand opening of a Randolph-Brooks Federal Credit Union branch in Pflugerville, Texas, the parking lot was so full that customers had to leave their cars across the street.

Dozens...