I'll take the operational Army as an example. Infantrymen, for example, who live on the FOB and have nothing better to do at night than lift weights get very big. They get regular, nutrituious meals, and despite the fact that their opeartions may be stressful, they retain the body mass, specifically, muscle mass, quite readily.
Infantrymen who work in Afghanistan, on the other hand, where long days, MREs and exposure to the elements is the norm, are losing weihgt at record levels. These guys probably take in more calories than the FOB-based operators in Iraq, but they pump out a lot more, too. And the Army rations by person, not by size or muscle mass.
Bottom line, small people do better in those types of environments. SF guys are not huge, they are usually small framed and very cut. Lance Armstrong, not Ahnuld. I hate to say it, but that is a big reason I stayed away from the Infantry: I realized a long itme ago that I was not built for it. 6'3" and 240 is great if you can feed it and you don't have to move it to far, but when it comes to dismounted ops, the extra size only hurts.
Being big does help when dealing with Iraqis and out of control soldiers, though. It helps a lot.
Hooah.