Let's say a vendor wants to increase prices but feels like they can't get away with it. Maybe their product has already gone through several recent price increases (this can happen when there is a shortage of a commodity like coffee or orange juice) or their is a recession.
In situations like these the vendor may be reluctant to visibly raises prices by putting a bigger number on the sticker. Instead they try what is known as a "stealth" increase, one that they hope you won't notice. They do this by reducing the amount of product in the package and hoping you won't notice.
With toilet paper, paper towels and tissue the practice is referred to as de-sheeting – the vendor simply puts less sheets in each package. With things like coffee it is referred to as "Weight-Out", and this is why you see thirteen ounce cans of coffee instead of the one pound can that was once standard.
Here is a real world example. For over a decade a large grocery store sold frozen shrimp in standard one-pound (454 gram) packages as shown below.
Recently they de-weighted the package, reducing the contents to 400 grams:
Assuming that the price stays the same this is a price increase of over 12%. Even if they also reduced the price – say from $13.99 to $12.99 – there is still an effective increase of over 5%.