There are a good number of fat boys and girls in the reserve components, and even some fat generals on active duty :-) As stated above, reservists, including guardsmen have to maintain their fitness on their own time, where as the active military gets time during the normal duty day.
Until just recently, the US Army Reserve was more concerned with total troop strenght and so quality, as far as health and physical fitness has slipped. With recent lessons-learned from Iraq, where reservists and guardsmen were having heart attacks, strokes, etc., in the 145-degree heat, slackards are being put out of the service. They have now realized that quality is more important than quantity, especially if the quantity will be culled out at the mobilization and spend the war being medically processed!!!
I'm lucky, since I work full-time for the Army Reserve I get time at work for fitness training and we have a gym to boot.