when I was growing up, this psyop was in our school books

[Honest]

the idea was "if we can convince people to eat low calorie density foods, then they won't be as fat". that's why they also recommended drinking an insane amount of water and eating an impossible amount of fibre. that's also a reason why pufa/mufa was recommended over sfa. 

and now government is recommending higher protein for the same reason.