If you work for a small employer, especially if your co-workers have small account balances, your employer may have a harder time driving a bargain with the financial firms, often called "record keepers, " that administer 401(k)s.

WSJ: Is Your 401(k) a Good Deal?

Some of the funds appear in 401(k) plans or are sold through financial advisers, and part of their expenses pay for costs to administer the plan or compensate advisers.

WSJ: Upside: Don't Pay High Fees for Index Funds