Those numbers are the envy of proponents of annuitizing 401(k) assets, who are fighting an uphill battle in the effort to create guaranteed income streams from plan savings.

Annuities occupy little space in 401(k) plans. According to the Employee Benefit Security Administration, less than 1 percent of all workplace defined contribution plans even offer annuities in investment lineups.

The disparity is not necessarily because sponsors of 403(b) plans take the paternalistic approach to plan design that many advocates of retirement security wish 401(k) sponsors would take.

Nor is it necessarily due to the non-profit sector being more politically and culturally aligned with the defined benefit culture of days past.

Mike Ericson, a research analyst at LIMRA, which tracks annuity sales, cautions against comparing the primary savings options in the for-profit and non-profit world.

“There was a time when 403(b) savings plans were completely annuitized,” he explained.

That changed with the passage of the Employee Retirement Security Income Act, he said, which allowed sponsors to begin offering mutual funds to 403(b) participants.

The result has been a steady shift in assets from annuities to mutual funds. By 2012, mutual funds accounted for 47 percent of all 403(b) assets.

As mutual funds’ share of the pie has grown, so have the type of funds offered. About 43 percent of assets in mutual funds where in equity-designed products, with 8 percent being in balanced target date funds, and another 8 percent in bond funds.

By comparison, TDFs make up about 22 percent of all assets in the 401(k) world, according to Northern Trust’s defined contribution tracker.

Index funds, favored by cost-conscious administrators, are also on the rise in 403(b) plans, representing about 14 percent of all 403(b) assets.

That helps explain continued lowering of overall costs to participants. In the 2012 the average 403(b) plan paid 75 basis points in total costs, down from 80 in 2009.

As is in the 401(k) universe, smaller plans suffer from limiting cost efficiencies. Participants in plans with less than $1 million in assets paid an average expense ration of 81 basis points on domestic equity mutual funds, compared to 48 basis points for participants in plans with more than $1 billion in assets.