For U.S. mutual fund investors, this is shaping up to be a year when it pays to go small.

In what mutual fund experts call a rarity, tiny funds with less than $100 million in assets under management are either leading or are among the top three or four best performers in every major U.S. stock category tracked by Morningstar for the year through June 10.

The outperformance of miniscule funds run by managers that few investors have ever heard of likely reflects the fact that, as passive investing in index funds and exchange traded funds becomes more popular, fund managers with few assets are taking concentrated bets on only a handful of stocks in order to stand out.

When all goes well, that can lead to strong outperformance even after a small fund's higher-than-average fees; when it does not, those funds are likely to fall among the worst-performers.

At the same time, stock pickers tend to reap the biggest rewards in the later stages of a bull market, when rallies are less broad.

Each of the small funds leading their categories this year have 30 or fewer stocks in their portfolio. The $25 million Biondo Focus fund, for instance, has gained 9.8 percent for the year with its portfolio of 19 stocks, trailing only two other funds among the 1,743 in the Morningstar large-cap growth category.

Its top holdings include a 14 percent stake in JPMorgan Chase & Co, a 12 percent in Pacira Pharmaceuticals Inc, and 10.3 percent in Gilead Sciences Inc.