Some investors prefer to invest in small-caps because of their stronger growth potential. When companies are smaller, they have greater room to grow than companies that are already very large. This can produce significant investment outperformance, particularly during bull markets. Of course, the opposite is that small-caps tend to underperform large-caps during bear markets.

The U.S. equity market is in a prolonged bull market since the market lows of 2009—the heart of the financial crisis and ensuing market downturn. As a result, investors may be enticed by the performance potential of small-caps. For investors looking to gain greater exposure to small-caps, here are several top-performing small-cap mutual funds.

Focusing on small-cap mutual funds, investors can choose among a large number of funds that have performed well over the past few years, with the added advantage of low annual fees and expenses.

First, the Fidelity Small Cap Growth Fund (FCPGX) has returned 10% year-to-date, and has a 17% average return over the past three years. It carries a 0.92% expense ratio and earns 4 out of 5 stars from Morningstar mutual fund ratings. Its top 10 holdings comprise 17% of its total portfolio, which indicates this is a highly diversified fund. Some of its biggest holdings include the iShares Russell 2000 ETF, Starz and Vail Resorts.

Second, the Wasatch Small Cap Value Fund (WMCVX) has returned 4% so far this year, but it has returned 16% on average over the past three years. Its annual expense ratio is 1.2%—still below the average expense ratio in its peer group. Like the Fidelity fund, the Wasatch fund has also earned 4 out of 5 stars from Morningstar ratings. Among its top holdings are Skechers USA Inc, Nu Skin Enterprises and LGI Homes.

And lastly, investors could consider the Nuveen Small Cap Value Fund Class A (FSCAX). This fund has returned 8% year-to-date, and 16% per year on average over the past three years. It has a higher expense ratio than many of its peers, at 1.4% annually. However, one distinction for this fund is that it offers a 0.4% dividend yield. Many small-cap funds do not make dividend distributions, so this fund may be more attractive to income investors who desire yield. Among its top holdings are MFA Financial, CNO Financial Group and California Water Service Group.

Since the Great Recession and brutal bear market of 2008, stocks have returned significant gains. Small-caps have led the way. Going forward, the outperformance of small-caps versus large-caps could continue if the global economy keeps steadily recovering. Investors who purchase mutual funds are not confined solely to large-caps. There are a number of mutual funds focused on small-caps that have provided strong returns over the past several years.