That shift might have been plausible if it happened in the 40s or 50s when the economy switched from war to consumption - but in the 80s? What kind of massive breakthrough in food production happened there that we mysteriously never heard of?

Food as a % of income declined dramatically. This chart has it at 18% in 1960 declining to ~10% today.

https://www.ers.usda.gov/amber-waves/2020/november/average-s...

There were a ton of programs after WWII to improve the nutrition of the country. This largely meant raising calories to prevent malnutrition. And the 80s are as good a point as any other to where that succeeded.

Aspartame