Companies hate raising prices, or more specifically customers hate it when prices are raised so companies don't like to do it. especially in foods. As such the go-to dance to keep up with inflation is to reduce the size while keeping the price the same as long as possible, when this becomes too noticeable the size is returned to original the price is increased and a big 20% more is added to the package. Repeat as needed.

Note that this is the most charitable interpretation where the customer deception is fairly minor(inflation happens, gotta sneak that price increase in there some how) Stuff like deceptive packaging(make it look like there is more than is actually present) punts it into the outright evil category.