The country’s biggest mutual funds are sucking up more and more of investors’ cash. But in many cases, investors aren’t getting much bang for their buck, Smart Money reports.

The country's 100 biggest funds bask in almost $3.5 trillion of assets. And the average size of those funds has soared by 62 percent since 2008.

Historically, the funds have grown both from attracting new investors and from superior investment returns. But now that the funds have grown so huge, returns are dwindling.

Editor's Note: Obama’s Economic ‘Fix’ is In . . .

The average fund in the top 100 outperformed only about 40 percent of its peers over the past three years, according to the magazine’s analysis.

Over the last five years, the Growth Fund of America, the third largest in the country, has lagged behind 77 percent of its competitors.

"There are no benefits to a fund being bigger. Success breeds sloppiness," says Seth Lipner, a securities lawyer and law professor at Baruch College.

Part of the problem is that the larger a fund grows, the more difficult it is to stay nimble. The fund needs to buy a huge amount of stocks or bonds to hold a meaningful position.

And the universe of securities in which a huge fund can buy en masse without distorting the market is small.

The other issue is that fund managers are greedy for fees, and the bigger the fund, the bigger the fees, regardless of performance.

Editor's Note: Obama’s Economic ‘Fix’ is In . . .