While I've no doubts shrinkflation explains a great deal of this, it would be interesting if increased mandatory food content reporting on packaging had even a small effect on reducing the calorie content of confectionery over time, as consumers could more easily compare etc and market responded with products in portion sizes to better fit preferences.
You can even see in the article photo, the more recent modern Mars bar has the calorific content printed on the front, in addition to the big label all UK food has on the rear. The 1991 example does not.
https://en.wikipedia.org/wiki/Big_Gulp
Even bigger sizes arrived in the 1980s:
Called Big Gulp because it was initially the largest such drink available at any retailer, 7-Eleven eventually introduced larger sizes. In 1986, they introduced the 44-US-fluid-ounce (1,300 ml) Super Big Gulp, followed by the 64-US-fluid-ounce (1,900 ml) Double Gulp in 1989 (later reduced to 50-US-fluid-ounce (1,500 ml))...
"Super Size" was a 90's thing as well, if I recall. It was basically an arms race of which company could provide the most ridiculously large portions of a cheap product like soda or fries.
Then the PR swung in the 00's due to the obesity epidemic and such stuff was silently rolled back. Likely for both PR and cost reasons. The 32oz drink stuck around though pretty much everywhere.
I only moved to the US ~10 yrs ago and quite unfamiliar with US in 80s/90s
It's an easy thing to promote more of to divert attention while reducing something else that actually costs money to make.
Or why portions of fries go up when burgers go down in size. Cheapest food item.