“There’s definitely a global race to become the biggest and survive,” said Mr. Phillips, professor of finance and business economics at USC. Mr. Phillips says mergers tend to result in greater corporate profits—generally bad news for consumers because the benefit of the merger isn’t being passed along—as well as an increased number of product offerings, which is generally good for consumers because it reflects innovation.

Messrs. Hoberg and Phillips’s data examines how dominant companies are in the customized markets they have carved out over time. To do so, they analyzed public filings to identify each company’s product lines and associated revenues, then calculated customized Herfindahl-Hirschman Index scores, the same measure used by the U.S. Justice Department in antitrust analysis.

HHI scores are calculated by squaring the market shares of participants in a market and then adding them. A company with 40% of sales in a market would have an HHI of 1,600 for that market, while another company with 35% of sales would bring the combined total to 2,825. Recognizing the U.S. economy had become more concentrated, the Justice Department in 2010 raised the threshold for what it considers highly concentrated to 2,500 from 1,800.

Whereas regulators determine market-share for specific products or services, the researchers do so for a company’s individual combination of businesses. So the USC scores would likely differ from HHI scores calculated for merger reviews.

Overall, among more than 1,700 public companies operating in 1996 and 2013, 62% had a bigger share of the markets in which they competed at the end of the period than at the beginning, the USC data show. For example, half the public companies in the consumer staples sector in 2013 had an HHI for their products above 2,893, up from 2,661 in 1996.

In addition to mergers, the pattern may reflect a decision by companies to jettison business lines with more competition, to focus on those they can expect to dominate, Mr. Phillips says. So one electronics firm may avoid competing head-to-head with another by specializing in specific kinds of components, for example.

The increasing concentration is a reflection of how much more easily companies can access large, global markets and gain economies of scale, said Mark Cooper research director at the Consumer Federation of America. “The question of how many firms there should be changes with the nature of the economy,” Mr. Cooper said.

In the grocery business, large firms like Kroger Co. and Wal-Mart Stores Inc. are massive compared with rivals. In 1996, 40 publicly traded companies operated food retailers, according to S&P Capital IQ. As of this year, only 18 do.

Jonathan Weis is the CEO of one of those players, Weis Markets Inc., based in Sunbury, Pa. Mr. Weis says that while the grocery industry still has many smaller regional and independent companies, it has become more difficult for mom-and-pop shops to stay in business.

“We are seeing more and more sales of independents who are just unable to compete or are ready to cash out,” he said. Weis Markets, which generates nearly $3 billion in annual revenue from 163 stores, has acquired around eight smaller grocers in the past five years, Mr. Weis said.

Census data suggest that even considering foreign and closely held firms, the grocery business was growing more concentrated through at least 2007, the most recent year for which data are available on retailers. That year, the four biggest grocers accounted for 31% of U.S. sales, up from 20% in 1997.

Consolidation has left the wireless industry dominated by just four national players, and even there, AT&T Inc. and Verizon Communications Inc. tower over T-Mobile US Inc. and Sprint Corp. The two smaller companies combined have fewer customers and generate less revenue than Verizon alone. The four carriers hollowed out the industry’s midsection in recent years as AT&T bought Leap Wireless and T-Mobile bought MetroPCS.