Ms. Stauber, 33, said she began inspecting her other purchases, aisle by aisle. Many canned vegetables dropped to 13 or 14 ounces from 16; boxes of baby wipes went to 72 from 80; and sugar was stacked in 4-pound, not 5-pound, bags, she said.

Five or so years ago, Ms. Stauber bought 16-ounce cans of corn. Then they were 15.5 ounces, then 14.5 ounces, and the size is still dropping. “The first time I’ve ever seen an 11-ounce can of corn at the store was about three weeks ago, and I was just floored,” she said. “It’s sneaky, because they figure people won’t know.”

In every economic downturn in the last few decades, companies have reduced the size of some products, disguising price increases and avoiding comparisons on same-size packages, before and after an increase. Each time, the marketing campaigns are coy; this time, the smaller versions are “greener” (packages good for the environment) or more “portable” (little carry bags for the takeout lifestyle) or “healthier” (fewer calories).

Where companies cannot change sizes — as in clothing or appliances — they have warned that prices will be going up, as the costs of cotton, energy, grain and other raw materials are rising.

Image Nabisco's Fresh Stacks package of saltines, top, contains about 15 percent fewer crackers than the old package.

“Consumers are generally more sensitive to changes in prices than to changes in quantity,” John T. Gourville, a marketing professor at Harvard Business School, said. “And companies try to do it in such a way that you don’t notice, maybe keeping the height and width the same, but changing the depth so the silhouette of the package on the shelf looks the same. Or sometimes they add more air to the chips bag or a scoop in the bottom of the peanut butter jar so it looks the same size.”