Larger portion sizes, more snacking, much less manual labor. Only the wealthy were obese back then.

It's an intriguing question! People of the early 20th century ate too much fat, too much sugar, too many carbs, too many calories, and yet did not grow obese. There is an interesting paper [1] making a strong case that the cause of the 21st century obesity epidemic is an unidentified environmental contaminant which (1) causes obesity in both humans and other mammals; (2) accumulates in major river valleys; (3) started noticeably affecting the US around 1980, and by now has spread worldwide.

Some possible candidates proposed for such a contaminant include animal antibiotics, PFAS, and lithium (e.g. in industrial lubricants).

[1] https://www.lesswrong.com/posts/6miu9BsKdoAi72nkL/a-contamin...

Troops consuming 5000 calories a day seems reasonable for walking (and occasionally running) across Europe with a pack and a gun.

a lazy person in the 20s would probably seem fairly active by today's standards... outside of reading books you would have been hard pressed to find an abundance of things to do while sitting around

Logically then, the stuff they ate wasn't the cause of obesity. Modern nutrition must have got it wrong.

Seed oils.

Given how much agricultural productivity has improved, I wonder if these descriptions of meals are more aspirational than everyday.