Because that would fit a lot of people's perception of how food is going: you get less, but what you do get is worse for you.
For example, vegtable oil instead of olive oil, fructose corn syrup instead of honey or sugar.
Beyond that chainging composition to reduce amount of expensive ingredients. Think Pesto Genovese which is originally Parmesan, Basil, pine nuts, etc. Instead of only pine nuts, you now include other nuts for texture and only add a smaller amount for flavour (and worse add flavoring so customers don't notice too quickly).
It's conceivable that if they've removed better quality ingredients, calories per gram of weight could go up.
That's obviously hard to measure because you can't do a blind taste test of a 20 year ago candy bar and one today, and people's tastes change as they get older. But worse ingredients making a worse flavor is possible too