There's a concept called 'tooth-to-tail ratio' which is the ratio of combat personnel to non-combat personnel in an army. The modern US army has a tooth-to-tail ratio of about ~1:8. This isn't necessarily a great correlation to physically demanding versus not-physically-demanding jobs, it does illustrate how tiny the front-line portion of the military actually is.

That was the concept I was gesturing at but I couldn't remember the name. Militaries in general but the US in particular are predominantly logistics organizations with a lethal last mile delivery focus. It's part of why they're so good for responding to natural disasters in far flung island countries, they're used to delivering goods with little infrastructure at the destination.