You need to pay more attention then, it's been nick-named the 'Grocery Shrink Ray' and practically all companies are doing it now. The strategy is basically either change the shape of the container (smaller) and keep the same price or to raise the price. Here is a good link to follow;

http://consumerist.com/tag/grocery-shrink-ray/

Oh, and my favorite one yet, "Now with 25% more!" while not actually offering any more.