ABC News' Rebecca Jarvis and Michael Koenigs report:

Before heading for the register, consumer experts are urging customers to take a second look at their items or chance paying the same price for less.

Edgar Dworsky of Consumer World said savvy shoppers should always look at the price per ounce rather than the price per package.

"Whenever a manufacturer says 'new and improved'… maybe they've taken out some ounces," Dworsky told ABC News.

For example, when Kristin Quinn of Boston, the blogger behind Misadventures in Mommyhood, went shopping with her family, she found that Minute Maid juice boxes had gone from 6.75 ounces to 6 ounces but the price has stayed at $3.49.

"This is not something that would ever come on my radar unless somebody pointed it out! Never!" the mother of two said.

Minute Maid told ABC News in a statement: "We are not immune to adverse impacts on our brands such as increasing commodity, fuel, packaging, and fruit costs. Earlier this year we introduced the 6 oz. package size for our juice boxes. The adjusted size allowed the business to successfully maintain retail prices amidst rising input costs while still providing preferred sizing for the category."

Other major brands are also downsizing.

Keebler Town House Crackers went from 16 ounces to 13.8 ounces and Keebler Chips Deluxe went from 14.5 ounces to 11.3 ounces.

Kellogg, which makes Keebler cookies, responded in an emailed statement, saying: "As commodity prices and other costs increase, Kellogg occasionally adjusts package sizes and wholesale prices. Ultimately, the price paid by the consumer is determined by the grocery retailer."

And these decreases are not only popping up in food. A package of Pampers diapers that used to hold 100 diapers now holds 92. Yet it's remained the same price: $25.99.

Procter & Gamble, which manufactures Pampers, said in a statement: "When our costs increase, we make every effort to offset costs and minimize the impact on consumers by increasing the efficiencies of our own processes, but some of it may need to be reflected in either the price or size of the product."

As prices of products increase, be sure to check the price per ounce before checkout.