Getting bigger could ease some challenges facing Ahold and Delhaize. It also may magnify their problems.

Shares of the Europe-listed food retailers rose 13% and 5% Monday, respectively, on reports they are discussing a deal. Both make about 60% of sales in the U.S., and sit in the squeezed midmarket. Whole Foods Market and Trader Joe’s have gained in natural foods while Wal-Mart and Costco compete on price.

Ahold...