Large-cap stock mutual funds and the S&P 500 have traced very similar paths in the past 10 years, and neither as lucrative as those of small- and midcap mutual funds. But large caps are flexing their muscles this year. The average large-cap mutual fund, seeded with a $10,000 investment on June 30, 2005, would have brought in $21,765 for investors as of Aug. 17 this year, a bit behind the $21,844 for the S&P 500, according to Morningstar Inc.