Shares of discount brokers are gaining the most since 2003 compared with the Standard & Poor’s 500 Index, a sign that small investors are joining the four-year bull market even after U.S. stocks suffered their biggest losses in six months.

Charles Schwab Corp. (SCHW), TD Ameritrade Holding Corp. (AMTD) and E*Trade Financial Corp. (ETFC) have climbed 38 percent on average in 2013, beating the S&P 500 by 23 percentage points and eclipsing returns in financial shares from Goldman Sachs Group Inc. to Bank of America Corp. (BAC), according to data compiled by Bloomberg. The last times that happened, equity mutual funds received about $91 billion, 24 percent more than the annual average in the two decades before the financial crisis, the data show.