There's safety in numbers, the old saying goes, and when it comes to mutual fund investing, it's not a bad principle. The largest mutual funds have trillions in assets under management (AUM), in addition to lower expense ratios, which may improve performance over time. In addition, the biggest mutual funds provide access to premiere money managers who specialize in maximizing your investments on a very granular level (though they'll also charge you fees for this upkeep).

Currently, two companies dominate the domestic mutual fund market: Vanguard and Fidelity. Both offer very robust funds with high growth potential and have trillions under their belt in total assets. If you're looking to cash in on the potential advantages of size in your mutual fund investments, here are the five largest mutual funds as of December 2020.

1. Vanguard Total Stock Market Index Fund Admiral Shares (VTSAX)

  • Assets under management: $921.4 billion
  • Expense ratio: 0.04%
  • 1-year performance: 19.14%
  • 3-year annualized performance: 13.21%

For investors willing to invest a minimum of $3,000, the VTSAX fund provides exposure to the entire U.S. equity market: small-, mid-, and large-cap growth and value stocks. Created in 1992, the Vanguard Total Stock Market Index Fund Admiral Shares has almost a whopping trillion in assets under management, with more than 3,590 stocks in its holdings. Its portfolio leads with Apple, Microsoft, Amazon, Alphabet, and Facebook.

2. Fidelity 500 Index Fund (FXAIX)

  • Assets under management: $274.1 billion
  • Expense ratio: 0.015%
  • 1-year performance: 17.44%
  • 3-year annualized performance: 13.16%

Topping off the list is Fidelity's large-blend fund, tracking the S&P 500. The top 10 holdings in this portfolio make up 27.86% of its portfolio, including Apple, Microsoft, Amazon, Facebook, and Alphabet among others. However, FXAIX's expense is one of the lowest in the market and maintains a solid 4-star Morningstar rating.

3. Vanguard Institutional Index Mutual Fund (VINIX)

  • Assets under management: $231.6 billion
  • Expense ratio: 0.035 percent
  • 1-year performance: 17.44%
  • 3-year annualized performance: 13.14%

VINIX tracks the performance of the S&P 500 Index, which tracks large-cap stocks. This is a passively managed mutual fund with a low expense ratio; however, the minimum investment required here is $5 million, making it a pricy option for the average investor. It's no wonder its total assets under management are around the $231 billion mark.

4. Fidelity Government Cash Reserves (FDRXX)

  • Assets under management: $210.4 billion
  • Expense ratio: 0.38%
  • 1-year performance: 0.39%
  • 3-year annualized performance: 1.25%

This mutual fund is extremely low-risk: similar to Vanguard's equivalent fund below. In fact, 42% of FDRXX is invested in U.S. Treasury bills and strives to maintain a stable share price of $1. With this fund, although having a huge asset pool, investors could lose money by investing in the fund as opposed to investing in a more aggressive stock-forward strategy.

5. Vanguard Federal Money Market Fund (VMFXX)

  • Assets under management: $203.9 billion
  • Expense ratio: 0.11%
  • 1-year performance: 0.58%
  • 3-year annualized performance: 1.49%

Vanguard's Federal Money Market Fund, VMFXX, is one of the company's most conservative funds, despite punching in the heavyweight category with $203.9 billion assets under management. This mutual fund is all about stability: the fund invests at least 99.5% of its total assets in cash and short-term U.S. government securities.