Diets have improved where they have and not necessarily in the ways after the semicolon. The diets followed by 300+ pound people are unlikely to be healthy. Also, a lot of “light” foods are about as light as my laptop’s (black) keyboard; they substitute one source of calories for another but they’re not either less caloric or healthier with regards to other nutrients than the equivalent non-light food.
Yes, exactly, and the study DSeid linked to was studying how people’s opinion changes, and so of course they’re not controlling that away.
You should check out “Nutrition and Physical Degeneration” by Weston A. Price.