For investors who can bear to peek at their 401(k)s, yearend is the time to reassess portfolios. To find out which actively managed mutual funds are the most popular, Bloomberg Businessweek asked independent 401(k) ranking service BrightScope to screen its database, which covers 50,000 plans—including 90 percent of all assets in 401(k) plans—and uses the most recent plan data, ranging from yearend 2007 to yearend 2009. The results listed some funds, such as large-cap Fidelity Contrafund (FCNTX ), that reward investors with good risk-adjusted returns. In Contrafund’s case, active management lifts it above the crowd. Others, such as small-cap growth fund Vanguard Explorer (VEXPX ), are underperforming their benchmark indexes. Here’s a look at the popular kids in three fund categories.

1. Large-Cap Stocks

American Funds’ $151.3 billion Growth Fund of America (AGTHX ) dwarfs the No. 2 fund, $68 billion Fidelity Contrafund. Growth Fund, whose nine managers track 283 stocks, has energy as its largest sector (7.2 percent), and Oracle (ORCL ) as its top holding. As assets have swelled from $37.7 billion in 2000, so has its overlap with the S&P 500, according to Morningstar (MORN ), as measured by active share—a gauge of overlap between fund holdings and a benchmark index. Growth Funds’ overlap rose from 33 percent in September 2005 to 45.7 percent this past quarter (Contrafund’s rose from 23.8 to 26.2). Contrafund, managed by William Danoff, has 472 stocks, and 32 percent in technology. (Top holding: Apple (AAPL ).) It consistently outperforms Growth Fund.

2. Mid-Cap Stocks

Fidelity Low-Priced Stock (FLPSA ) heads up mid-cap funds, with a 10-year annualized return of 11.5 percent, vs. 6.8 percent for runner-up T. Rowe Price Mid-Cap Growth (RPMGX ). Fidelity’s $30 billion fund holds 907 stocks while rivals hold from 76 to 365. A third of assets are outside the U.S. in companies like the Canadian grocer Metro and U.K. retailer Next. Another widely held fund, Fidelity Mid-Cap Stock (FMCSX ), is up 11.6 percent this year. Its 17.3 percent stake in financials (as of Aug. 31) has depressed returns. Mid-Cap, which tends to hold cyclical plays, aims to be a core fund, but has a growth bent, says Morningstar analyst Christopher Davis. If you want all your mid-cap exposure in one fell swoop, I’m not sure this fund can do that.

3. Small-Cap Stocks

The top pick: Neuberger Berman Genesis (NBGNX ). The $10 billion blend fund has a 10-year annualized return of 10.6 percent, vs. 3.7 percent for the No. 2 fund, Vanguard Explorer. The top holding among its 126 stocks: salt and potash producer Compass Minerals (CMP ). Explorer, which holds 659 stocks, trails the Vanguard Small-Cap Growth Index (VISGX ) over 10 years. (Explorer uses the Russell 2500 Growth Index as its benchmark.) Daniel P. Wiener, editor of newsletter The Independent Adviser for Vanguard Investors. notes that Explorer’s number of holdings has been cut by 40 percent over the last 12 months. Even so, he says, Explorer, managed by six sub-advisors, has too many cooks in the kitchen and the meal they’re making isn’t particularly tasty.