Doritos, and Tostitos—will sell “bonus bags” with 20 percent more chips than standard bags, for the same price. It will also add two extra bags to its single-serving variety pack.

PepsiCo is trying to win back customers turned off by “shrinkflation” with a special promotion that offers more potato chips in bags of Lay’s, Doritos, Tostitos and Ruffles. The company told ...