And that is providing new opportunities for investment managers, particularly those managing ETFs, with have been slow to penetrate the 401(k) market, even as they post record numbers of adoption through retail channels.

Mega sponsors cited access to a broader range of products, like ETFs, offered through managed accounts as a key reason for adopting managed platforms. ETFs and other indexed products feature low expense ratios, which could offset the added cost of a managed platform, which purport to give participants more nuanced guidance than a standard target date strategy.

In April, Fidelity reported its managed-account client base grew by 29 percent in 2014, with 790 new sponsors signing on to its platform, translating to 163,000 new participants.

Cerulli Associates, a Boston-based research firm, is expecting the managed account market to hit $6.7 trillion by 2017. As of June 2014, assets in all 401(k) plans totaled $4.4 trillion, according to the ICI.

That would be a notable benchmark. Last year, Cerulli said 401(k) managed accounts held about $108 billion in assets. The Plan Sponsor Council of America says that 36 percent of all employers with defined contribution plans offered managed accounts in 2012, up from 25 percent in 2005.

The latest reports of large-sponsors' interest in managed accounts comes in the wake of a report released by the Government Accountability Office last year, much of which was critical of the platforms.

The GAO looked at eight unnamed providers of the accounts and found a wide discrepancy in the fees they charge, ranging between $8 and $100 on every $10,000 invested.

In spite of the GAO's criticisms, Cogent's report found that the fiduciary protections featured in managed accounts are also enticing larger-sponsor interest.

Charles Schwab's Investment Management platform was the largest provider to the biggest plans, with Goldman Sachs, Fidelity, BlackRock and Vanguard rounding out the top five.