When it comes to an index like the S&P 500 where even some of the smaller components (e.g. Church & Dwight Co (read: Arm & Hammer), Hilton, etc) would be consid

When it comes to an index like the S&P 500 where even some of the smaller components (e.g. Church & Dwight Co (read: Arm & Hammer), Hilton, etc) would be considered "large cap" then you probably want to do equal weighting rather than market cap weighting. When it comes to something like Vanguard's Total Market, you probably want to put more money into the blue chip stocks than into the smaller companies at the bottom of the list (sorted by market cap). Smaller companies have lower trade volume/are less liquid which makes them subject to greater volatility/price fluctuations. There is no sense taking on greater risk there when the reward opportunity doesn't meaningfully increase with it.