Vanguard, the biggest U.S. mutual fund company, is going after small fish it once passed up.

Since 2012, the Malvern, Pennsylvania-based fund firm has quadrupled to roughly $4 billion the assets it administers in retirement plans holding less than $20 million in employee savings -- a market it once dismissed as a backwater.

Vanguard joins rivals like T. Rowe Price Group Inc. in putting a new emphasis on the micro-plans. Both trail Fidelity Investments, the sector's Bigfoot with $94 billion in assets from smaller companies.

The newfound interest reflects how the largest corporate retirement plans are either locked up or bargaining hard on fees, while many smaller companies have room to cut their costs and are under pressure to do so.

Among 20,000 plans with less than $20 million in assets tracked by BrightScope Inc, total costs averaged 1.18 percent in 2012 -- more than three times the total average costs of 0.33 percent for top corporate plans with more than $1 billion in assets.

Small companies are "where the worst plans are today," said Mike Alfred, BrightScope's chief executive. Since 2012, 401(k) plans have been required by the U.S. Department of Labor to clearly disclose fees to employees.

Company 401(k) plans, combined, held $3.7 trillion at the end of 2013, according to research firm Spectrem Group, up from $1.9 trillion in 2008. Of that, about $700 billion was in plans with fewer than 100 participants.

The money is widely scattered and just finding it takes work. In Lewisburg, Pennsylvania, Annette Camuso-Sarsfield, human resources director for Playworld Systems Inc, a recreational equipment maker, said she gets a call a month from financial representatives looking to run her $18 million plan, twice the rate of a few years ago. She hasn't changed plan administrators in recent years, but does review plan mutual funds and fees in order to remain competitive.

Vanguard began its small-business outreach in 2011, aiming to run the 401(k)s with the help of closely held Ascensus Inc, which runs websites and administrative services for the retirement vehicles.

Vanguard Managing Director Chris McIsaac said the effort reflects how low-fee Vanguard can underprice competitors -- and how it is harder to win new business from the largest corporations.