U.S. bankers who vowed last year to boost profit by shrinking expenses are finding costs don't always stay cut.

While spending is down at the biggest commercial banks, efficiency ratios—a key gauge of management's ability to control expenses—deteriorated at more than half of the 20 largest lenders during the fourth quarter, led by Bank of America Corp. and Citigroup Inc. Analysts including Mike Mayo at CLSA Ltd. are pressing them to do better this year, even as bankers say basic costs of doing business have gone up.

Efficiency is calculated by comparing operating expenses against revenues, which have been stagnant or shrinking faster than some bankers can find things to cut. At the same time, new costs are replacing old ones as regulators write more rules, mortgage investors bring more lawsuits, and customers demand more technology.