Performance by the average large-cap mutual fund has lagged behind small- and midcap funds over the past 15 years. But top-performing large-cap funds have managed to generate impressive gains. A $10,000 investment started in the average large-cap fund on June 30, 1999, would have grown to $17,315 by Aug. 19, according to Morningstar data. The same investment would have mushroomed to $34,753 in midcap funds and to $38,842 in small-cap funds.