But S&P 500 are not the 500 companies with highest revenue. Its 500 companies selected by a committee to be representative of the american economy. Its not as biased as you suggest.
The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock market exchanges; the New York Stock Exchange and the NASDAQ. Wikipedia
However, Mercury Interactive was included when it had around 1000 employees. 1000 is a lot, but you can still learn by comparing a company with 1000 employees to HP with 300,000.