VTI/VTSAX is 80% S&P 500 by market cap. You get slightly better diversification into mid- and small-cap stocks, but at minuscule weights that don't make much of a dent in the performance of the fund. In practice, there's little difference between holding VTI/VTSAX and, say, VOO/VFIAX.

(VTSMX is closed to new investors. VTSAX is the current fund.)