I reckon it's inflation control..chocolate bars etc all getting smaller no price change..magic, no inflation...lol, I noticed today that even Etch a Sketch are smaller more than when I had mine in the '60's...
In part, the trick seems to be to keep on reducing the size whilst maintaining the price and then bring out a bigger version at a bigger price and then start making it smaller. Walkers crisps seem to be the masters at it.
This it true for water but not other liquids.
So it looks more like the great H&S swindle, I'm not surprised, although those 1CWT bags of cement where a ******.