As stated by BT, its been happening for ages. Most of the products you purchase nowadays have been reduced one way or another, except in price.
I notice this a couple of years ago, when we purchased our usual brand of kitchen roll towels. The inner tube size had been increased, and the length of the rolls reduced, so making the previous holders redundant, and also the contents less.
Forgot to mention canned or bottled drinks. Check the content sizes of those products next time you make a purchase.
The other week I had to obtain Lucazade drink of 500ml for an hospital test. Going to the local supermarkets I was given the choice of 500ml or 1 litre bottles, plus containers of 4 or 6 bottles consisting of 380ml each. Checking the prices, there was hardly any real savings, but the 380ml worked out dearer (handy handbag or student size?).
Mars bars used to be a size that was used to compare all sorts of things with, because they stayed the same size for decades. For instance before the war you paid twopence each and got 120 mars bars to the pound. Inflation could be measured using that as a standard. But then a couple of years ago the size of mars bars was altered. So I have no idea what you can use now.
I'm just off to get one of the smaller sized ones out of the fridge for a treat... I may follow that up with a dollop of ice cream. (about the same amount you used to get in a twopenny wafer pre-war... 120 ice cream wafers to the pound as well? Can't be bad). Ice cream cornets were a penny each or 240 to the pound).