Passive investors love the $S&P 500(.SPX)$ . What they don’t realise is that the criteria for entering and exiting the index isn’t quite as passive as you would expect. The distinction between the S&P 500 large caps and the S&P 400 mid caps isn't always clear. Here I plot market cap vs revenue. It’s interesting to see that there are S&P 400 companies that have a larger cap and greater revenue than some S&P 500 companies.