People commonly lose weight during basic training in the US military, even as they are fed a terrible diet of extremely calorie dense processed foods. This is because basic entails consistent exercise from sun up to sun down, with extra exercise assigned as a form of punishment.

After having this experience, many servicemembers keep the mindset that diet doesnt matter as long as you exercise enough, so the military has a culture of excessive drinking, eating, and exercise.

The problem is that, unless your whole job is exercise, it is difficult to burn that many calories in a day - so you start gaining weight as soon as you get a desk job. And also, constantly pushing yourself like this is a good way to get an overuse injury, which many people gain as part of basic itself. And since it is difficult to train hard with an injury, you gain weight.

Also, I am gonna go out on a limb and say that this sort of culture is probably pretty universal across world militaries.