The jars have been the size they are for years, but over time the amount of chocolate powder in them has been reduced to such an extent that "but look, it says 200g on the label" is insuffient an excuse to cover the deception going on.
Here's a photo of an unopened jar with a 3000-lumen torch shining through it from behind.
The overall height of the jar is 162mm.
The top of the label is 119mm from the base.
The top of the chocolate inside is 33mm lower than that, 86mm from the base.
In other words, the chocolate powder occupies just over half the volume of the jar.
I may in general be a "glass half-full" person, but I don't want it to start out half-full when I buy it.
It was 250g in February 2020 when Sainsbury's told my wife she was the #5 buyer of the product in 2019, in an ill-conceived attempt at gamification.