And the trend looks likely to endure as the need to cut costs in retirement plans continues.

According to a Reuters analysis of U.S. Department of Labor reports, assets in CITs run by T. Rowe Price Group Inc skyrocketed 54 percent last year to $88.9 billion. Fidelity Investments' 16 CITs also saw a heady rise, gaining 31 percent to $85.6 billion.

At the same time, according to Morningstar Inc., actively managed funds saw a net withdrawal drain of $207.5 billion across the U.S. last year.

"What we have seen is a migration from higher cost mutual fund shares to what we call the i-class, and then to collective investment trusts," Scott David, head of T. Rowe's individual and retirement plan services, is quoted saying during the company's investor day last month.

The company's CITs may have gained lots of assets, but clients of T. Rowe Price made net withdrawals of nearly $6 billion from its actively managed funds last year.

Fidelity, too, saw hefty outflows of $47 billion in outflows from those funds. The firms are No. 4 and No. 2, respectively, in terms of U.S. actively managed mutual fund assets.

More outflows are expected this year; the Wisconsin Deferred Compensation Program, for example, plans to transfer about $685 million currently invested in Fidelity's Contrafund mutual fund to its cheaper Contrafund CIT. Both are run by stock-picker William Danoff.

The savings from the transfer, according to the report, would save nearly 20,000 participants currently invested in Contrafund about $550,000 a year in combined expenses; that amounts to nearly $30 per participant. The expense ratio would drop as much as 17 percent, according to retirement plan officials.

And that's just a single example. ALPS, a subsidiary of software and research firm DST Systems Inc., is cited in the report projecting that overall, assets in CITs could top $3 trillion this year; that's more than the growth of the overall retirement market.