this has nothing to do with calories you are harder when you work hard due to the fact that you will recruit your muscles in need and by that you will work more economically and burn less calories, that is one of aspects of training.

Also of course the body mass index plays a role because it will - amongst age and gender - have an influence of your basic calories consumption.

So you loose 20 kg body weight and your need for calories goes down as well, the usual Napoleonic soldiers was not fat at all, I assume a BMI of 19 to 20.

A pity that I cannot find any longer the study about calories and rations in the 18th century I once read.