I'm also surprised it's bigger by 20g, since obesity and being overweight wasn't quite as much common at the start of the 90s...

The reason is that back then while Mars bars were more unhealthy - everything else was more healthy.

Eg. Mayonnaise contained eggs and oil instead of the weird crap + sugar mayonnaise is made of now.

Also even things like the vegetables has more minerals in and the chicken had less fat in it.