The largest and most popular exchange-traded funds (ETFs) are those that track the performance of the S&P 500. In fact, the three largest ETFs as measured by assets under management are all ones that mimic the performance of this benchmark index. These funds include the Vanguard S&P 500 ETF (VOO -0.36%), the SPDR S&P 500 ETF Trust (SPY -0.33%), and the iShares Core S&P 500 ETF (IVV -0.34%).However, some investors have raised concerns about the current heavy concentration of megacap stocks that now dominate ...