The Star Tribune 100 is getting thinner at the bottom and heavier up top.

The 10 firms at the top of our list of the state’s biggest public companies brought in 79 percent of the revenue generated by the whole index in 2012. Ten years ago, their share was 71 percent.

Many factors have combined to thin the ranks of up-and-comers. Going public has become more difficult and costly, Minnesota’s crucial medical device start-up scene has gone fallow, and dozens of public companies in Minnesota have been acquired.

“We have a fantastic workforce and a lot of things going for us, but we’re just not cranking out that many public companies anymore,” said Bob Tunheim, a partner at law firm Lindquist & Vennum who handles mergers and acquisitions.

Ten years ago, more than 200 publicly traded firms were located in Minnesota, and median revenue for the top 100 was $456 million. The largest firm on the list that year, Target, had revenue of $48.2 billion. The smallest, Lifecore Biomedical, reported sales of $44.7 million.

Median revenue on the index has fallen 27 percent since then, to $329 million. The smallest is North St. Paul-based Aetrium Inc., which reported 2012 sales of $6.2 million. That’s just a bit more than what CEO Stephen Hemsley earned in cash incentives last year at No. 1-ranked UnitedHealth Group, which posted $110.6 billion in sales.

A key part of the story is acquisitions. Twenty-six of the 50 firms on the bottom half of the index in 2003 have since been acquired, including firms such as Lifecore, Retek, Stellent and Lawson Software.

Overall revenue on the Star Tribune 100 keeps climbing, to $474.2 billion. Firms like U.S. Bancorp, Target and General Mills posted solid gains. So did firms like UnitedHealth, 3M, Ecolab and Pentair, who went shopping in 2012 for companies in Brazil, California, Texas, Florida and Switzerland. By acquiring Brazil’s largest health insurer, UnitedHealth added roughly $5 billion in revenue, more than the combined annual revenue of the bottom 50 firms on the list.

“The big companies are in a great spot, and if you look at the balance sheets for a lot of these companies, they’re all doing swimmingly well,” said Rick Rinkoff, director of research for Craig-Hallum Capital Group. “The stock market, at least for big stocks, is doing great.”

This is partly because large companies are selling products to end users, while smaller companies often sell parts or supplies to larger companies, Rinkoff said.

“In this particular cycle, big businesses have a lot of power over small companies. They can say, ‘Hey, you don’t supply it at this price, I’ll go somewhere else,’ ” he said.

The decline in initial public offerings — when a company decides to sell shares on a stock exchange — is a national trend. The heady days of the late 1990s gave way to a drop in the 2000s and then a steep decline during the recession and slow recovery. Wall Street scandals inspired repeated layers of regulation, adding to the cost of being a public company.

The trend has been more pronounced in Minnesota. From 1996 to 2000, 28 Minnesota companies went public — an average of more than five per year, according to data collected by researchers at the University of California, Davis. In the past five years, six Minnesota firms made an initial public offering, and one of those was a real estate investment trust. BioAmber Inc. went public this month, the state’s first IPO for 2013.

“It’s the cost of going public, hassle of going public, cost and hassle of being public,” said Tunheim.

Going public was a more obvious choice back when St. Jude Medical did it in 1977, partly because there weren’t mature medical device companies around to snatch it up.

“You have to be much bigger and much more mature to go public these days than you used to be,” said Tony Carideo, a Minneapolis-based investment consultant. “A St. Jude Medical, at the size it was when it went public, could never go public today. It wouldn’t go public today.’’

Minnesota has about half as many public companies as it did 10 years ago, and it’s not as if venture capital and private equity are picking up the slack.

“There’s less angel and venture financing going into companies now than there was 10 or 15 years ago in the Upper Midwest,” Tunheim said.

Silver lining?

The dip in IPO activity should be good for smaller public companies, said Fred Martin, president of Disciplined Growth Investors. When lots of firms in similar industries are going public at the same time, they tend to fight with each other to the death. “When IPO activity is high, the small guys eat themselves,” he said. “You send 10 soldiers up the hill and nine die.”

As IPOs drop off, smaller public firms have less competition and should benefit. Another thing working in their favor has been, counterintuitively, the economic downturn, Martin said. Lean times force large companies to abandon peripheral business lines, meaning still less competition in some markets.

“When economic times are tough, big companies focus on their core competencies, and they stop this nonsense of going after hopeful markets,” Martin said.

Chris Puto, dean of the University of St. Thomas’ Opus College of Business, said he’s encouraged by the numbers he sees on the Star Tribune 100 index. The economic recovery has been unsteady, but revenue is growing overall and companies deep in the ranks are posting double-digit gains.

“The reason our state is doing well,” he said, “is because these companies are doing well.”