Investment-only assets, the study states, refer to defined contribution plan assets managed by an asset management firm. Proprietary plan refer to those run by a record-keeper or administrator.

Strategic Insight Research Analyst Bridget Bearden attributes the rise in market share to fee disclosure regulations that will “shed light upon what have often been considered ambiguous fee arrangements.”

The scrutiny of DC plan fees, the report adds, will speed the growth of “open architecture” DC plan models, notably among investment platforms that traditionally offered their own funds.

The total DC plan market—including 401(k), 403(b), 457, profit-sharing and similar plans—will grow to $5.9 trillion by 2017, the study says. This represents a 31 percent rise from the $4.5 trillion in assets posted at year-end 2011.

A survey conducted for the report found that mutual fund firms are responding to the DC investment-only growth opportunity. More than half of firms reported increasing commitment to DCIO by adding headcount: The median number of DCIO-focused sales people doubled to six in March 2012 from three in 2010.