It's a good option if you're looking to add small-cap stocks to your portfolio. But it's not your only good option.If you're looking to build a hefty war chest for retirement, it's a good idea to focus on stocks -- at least with your long-term investing.Check out the table, offering the returns of various asset classes between 1802 and 2021, per Wharton Business School professor Jeremy Siegel:Asset ClassAnnualized Nominal* ReturnStocks8.4%Bonds5%Bills4%Gold2.1%U.S. dollar1.4% Here's how your money could gro ...