One of the first items in which I noticed the downsizing, years ago, was potato chips. That 8 oz. bag has now shrunk to around 4 - 4.5 oz. (3.75 oz. for flavored chips). I recently noticed a game Tropicana Orange Juice plays: The regular oj is still in 64 oz. cartons. But the low acid one, which I buy, is now in 59 oz. cartons, as are all the other 'specialized' juices like calcium fortified, etc. The cartons look so similar in size that it is a deliberate deception. As mentioned above, another thing I've noticed the downsizing of is canned goods. I noticed it with the chicken/beef broth cans, down from 16 oz to 14.5 oz. Pretty soon, the prepacked food items will fit into a child's doll house and look quite normal.
I guess we're all going on a diet, whether we want to or not.