Food manufacturers have been changing package sizes to give you less product for your dollar for some time now. Not every food item is subject to this, but tuna's a classic example: a can used to be six ounces, now it's five. That's a 17 percent reduction in size...and you often see it on sale because, I think, manufacturers are trying to get us to accept the smaller size as normal, and...well, I think most of us have caught on to the fact that tuna's 'shrunk'.
You have to be careful to read labels on items as disparate as bleach (is it diluted, or full strength?) and fruit juice (although the amount of real juice in fruit juice has been an issue for a long time, it's worse now).
Packaging is also being changed to reflect a smaller amount being sold. Check the bottoms of containers to make sure that the outside isn't hiding a significant reduction via a wide skirt of air under the actual bottom of the container. If you notice a sale on a lot of an item, such as a spectacularly low price for canned goods, it might be an effort to clean out the old size, not just the old stock, to allow the new size to be shelved.
Be a smart shopper! And consider cooking from scratch when possible. It takes little more time to whip most things up from basic ingredients than use a mix; look at a box of cake mix, and you'll see it still requires eggs, and still requires beating, and while you might not have to measure out 2 1/2 cups of flour and a bit of baking powder, etc., it's barely faster to use the mix.