Both funds lack exposure to thousands of mid-cap and small-cap shares because the S&P 500 only contains 500 U.S. companies. When compared to either of these funds, VTI, for example, invests in 3935 equities, which is more than 3400 more stocks. Broader equity market indexes are more diversified and give investors exposure to an even larger, broader collection of stocks, even if SPY and VOO are both suitably diversified. For further diversification, I believe combining SPY and VOO with a few smaller investments in small-cap shares or equity funds would be excellent. However, it is perfectly legitimate for some investors to opt to concentrate on relatively safer large-cap equity or equity funds.