I recently picked up some name-brand orange juice. It was on sale for less than the store brand. Maybe I had one of the online coupons.
When I went back to get another half-gallon the next week, it was the same price as the store brand, so it should not have mattered which one I chose. I saw the two next to each other, and the name brand carton appeared thinner. I looked at the ounces in the carton and realized that the "half gallon" was actually 59 ounces.
Making packages smaller is not new. It has been going on for about five years. A "half gallon" of ice cream has been 1.75 qt for some time. It's how manufacturers avoid raising prices. Same price, less product. That's why we notice the cost of meat and produce going up, but not peanut butter.
Almost all grocery stores have a per-ounce cost calculated somewhere on the shelf tag. Ralphs was recently sued and found guilty of having incorrect ounce prices calculated. Aside from that, the unit price is what I use to decide which size package to get. Sometimes, the middle-sized can is a better value than the large one. It's rare, but it does happen, usually when only one size is on sale or has a coupon.
It's a shame that grocery shopping has become an algebra exam, but the only way to get the most for your money is to be diligent. Giving the kids a calculator and having them figure out value is a good way to keep them occupied during boring grocery trips and to teach them budgeting skills.