Are 401(k) participants putting their money into the best funds? Probably not, says Mike Alfred, CEO of BrightScope, a data firm that tracks 401(k) plans. After analyzing 50,000 retirement plans, Brightscope identified the 10 most popular funds chosen by participants. The list includes giant funds, such as American Funds Growth Fund of America ( AGTHX), which has $151 billion in assets, Fidelity Contrafund ( FCNTX), $68 billion, and Dodge & Cox Stock ( DODGX), $40 billion. These funds gained wide followings because they delivered solid performance but Alfred says the success may not last. "When funds become big, it is difficult for them to continue outperforming," he said. All too often, growing funds follow a disappointing pattern. In a typical scenario, a fledgling fund starts by delivering strong performance that attracts a big inflow of assets as investors discover the hot manager. As the fund expands, the portfolio manager must work harder to employ the assets. Instead of just taking a limited number of attractive stocks, the manager may be forced to sop up cash by buying some less appealing shares. With more assets in the fund, trading becomes more difficult and expensive. A manager who wants to buy $1 million worth of shares in McDonald's ( MCD) may complete the trade in seconds. By contrast, Fidelity Contrafund, which owns $1.6 billion worth of McDonald's, faces a monumental task. Buying billions of dollars worth of shares can take weeks or months. When the manager buys so much, the trading is likely to push up the share prices, making the transactions more expensive. All that drags down returns, as the nimble star fund turns into an unwieldy also-ran. Alfred would like to see the employers who sponsor 401(k) plans begin to select more small funds. But he said that isn't likely to happen any time soon. To pick funds, employers typically hire consultants who favor winning funds from big companies. The employers select a menu of funds, and participants tend to gravitate to those with the best track records.