THE BATTLE between the world’s two largest exchange-traded funds has reached a pivotal moment. On February 18th VOO, an ETF tracking the S&P 500 that is managed by Vanguard, a giant passive-investing firm, took the crown as the world’s largest. Days later SPY, an ETF managed by State Street Global Advisors, another giant, reclaimed it. Both funds boast assets of over $620bn.