BMI isn’t fatness. BMI is just weight / (height^2), which means that BMI grows linearly in proportion to a human’s scale. (Weight grows with the cube of dimensions but height^2 only grows with, well, the square.) BMI also fails to distinguish between muscle, bone mass, and fat. If, say, human heights had been increasing over time (while maintaining other bodily proportions), you’d expect BMI to increase over time.
That’s not to say humans aren’t getting fatter, but if you wanna hypothesize on the causes of trends in human fatness, you need data on human fatness. Not BMI.
Is the problem that our propaganda isn’t good enough, or that our progress isn’t good enough?