Vanguard’s blue-chip US stock exchange-traded fund has become the world’s biggest, seizing the crown from State Street’s S&P 500 tracker, in a sign of how a race to slash fees is reshaping the industry.

The Vanguard S&P 500 ETF (VOO) tracks the S&P 500 index. This index is seen as a measure of how the U.S. stock market is doing. Over time, the S&P 500 has given returns of about 9% to 10% each year. For example,