I was genuinely curious to see what the legally mandated calorie/ingredient content labelling was in 1991 on a UK Mars bar, but sadly no pictures of the underside.

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.