WASHINGTON—Four of the biggest meat-processing companies, using their market power in the highly consolidated U.S. market to drive up meat prices and underpay farmers, have tripled their own net profit margins since the pandemic started, White House economics advisers said. Financial statements of the meat-processing companies—which control 55 percent–85 percent of the market for beef, poultry, and pork—contradict claims that rising meat prices were caused by higher labor or transportation costs, advisers led by National Economic Council Director Brian Deese wrote in an analysis published on the White House website Friday. Officials studied earnings statements from Tyson Foods Inc., the chicken producer and biggest U.S. meat company by sales; Brazil-based JBS SA, the world’s biggest meatpacker; Brazilian beef producer Marfrig Global Foods SA which owns most of National Beef Packing Company; and Seaboard Corp. RIC. Those statements showed a 120 percent collective jump in their gross profits since the pandemic …