Small-cap stock funds have had the biggest returns this year, but is it too late to buy?

NEW YORK — The biggest returns this year have come from the smallest stocks.

Indexes that track small stocks are at record highs. Small- and mid-cap stocks have dominated their larger peers over much of the current bull market, which began in 2009.

Consider mutual funds that invest in high-growth stocks. Those that focus on small stocks are up about 21 percent so far in 2013, according to Morningstar. That beats the 15 percent return for large-cap growth stock funds over the same time.

To be sure, their strong performance also means that small-cap stocks no longer look like bargains. Prices for stocks in the Standard & Poor's 600 index of small stocks are trading at an average of 20 times their earnings per share over the last 12 months, according to data provider FactSet. That's higher than the index's average price-earnings ratio of 16 times over the last five years. It raises a key question for investors: Is buying into a small-cap stock fund now committing the investing sin of buying high?

Although it's clear that price-earnings ratios for small-cap stocks are less attractive than they once were, small-cap stock fund managers say these stocks can continue to climb. The economy appears to be strengthening, and when the economy has done well, so too have small-cap stock funds.

"The U.S. economy is like a rumbling giant starting to build up steam," says John Manley, chief equity strategist for Wells Fargo Funds Management. "It's not too late to buy small caps."

Here's a look at reasons why small-cap fund managers are optimistic, as well as some risks that investors need to consider before putting their money to work:

The most dynamic companies are small-caps, says Ralf Scherschmidt, who manages the Oberweis International Opportunities fund. It invests in small- and mid-sized stocks from around the world and returned 53 percent over the last year.

"Small-caps are inventing new products, where one or two new products can really change the fortunes of a company," Scherschmidt says. "For Procter & Gamble, it will be very difficult for one new product to move the bottom line."

Companies have just begun reporting their results for the second quarter and analysts are forecasting much bigger gains for small stocks. They expect the S&P 600 small-cap index to show 18 percent growth in earnings per share from a year earlier. That's more than quadruple the 4 percent growth that they expect from the S&P 500 index of large-cap stocks, according to S&P Dow Jones Indices.

Next year, analysts expect the gap to remain wide. They forecast 26 percent growth for small-cap stocks versus 13 percent for large-caps.

— MORE FOCUSED ON THE U.S.

The rest of the world has more question marks: Investors are worried about slowing economic growth in China, and the eurozone's economy is shrinking.

Such a dichotomy can mean good things for small-cap stocks, relative to their larger competitors. "By and large, you've got more of a domestic focus with small-caps, and that's the place you want to be right now," says Eric Mintz, portfolio co-manager for mid-cap and small-cap growth strategies at Eagle Asset Management. His $3.6 billion Eagle Small Cap Growth fund ranks in the top 20 percent of its category for 10-year returns.

— POTENTIAL DRAWBACKS

Although small-cap stock funds often do better than large-cap funds when the economy's healthy, they can also fall faster when conditions are souring. When markets flailed in the third quarter of 2011 -- after Standard & Poor's downgraded the U.S. credit rating in August and Europe's debt crisis was worsening -- small-cap growth stock funds lost an average 22 percent. That was a steeper drop than the 16 percent loss for large-cap growth funds.