Therefore, grains are cheap, everything is pumped full of salt and sugar, and people eat overeat.
Also, famines were semi-regular occurrences across the world until very recently.
Your idea would work if meat and eggs took fewer resources to produce, but reality does not work like that.
People only overeat themselves into obesity once you process those carbs into high fructose corn syrup etc. Seems like a very different problem.
There are regions of the world that are doing better than others, and a wide spectrum of reasons for that, but it is only comparative/relative improvement. Obesity is getting worse everywhere, across the board, as people are uplifted into middle class incomes and able to purchase and eat whatever they want & as much as they want.
It maps about 1:1 with the amount of sugary, fatty, addictive shit they're consuming. Across the globe. So do the trends. Prime example being the rising obesity rates in countries like Japan and Korea, rising at the exact same pace as the supply and consumption of above crap in those countries. They still have lower obesity rates than much of the West, at roughly the same relative difference of the amount of such crap consumed.