It also depends heavily on the sort of service you're set to do. I, for example, ended up in an administrative position. At the time, I was actually pretty fit, and while I didn't lose too much of my muscles or anything during the year, I did gain a lot of weight, 10-15 kgs, mostly fat.
You see, army food is rich in energy, meant to work well for those of us who had the most active duties. I had a pretty good appetite, as I had been pretty active up until joining the army. Add those together, and, well...