The concentration risk relates to diversification in investing. Index funds are generally thought of as a way to diversify a portfolio. Cap weighted index funds are generally preferred because they are cheaper for the provider to maintain. Compare VOO with RSV for example. VOO is cap weighted. RSV is equal weighted - which means investors in RSV bear the cost of periodically readjusting all holdings so they are once again equally weighted - something no necessary with VOO.

I am not the only investor who has taken steps to offset the overly high concentration in the SP500 that raises the riskiness of an investment portfolio. I've done so by splitting my VOO holdings in half, split 50/50 VOO/VTV that strategically diminishes the impact of the high top 10 stocks in the SP500.