Some of it is genetics on how easily your body develops during physical training. But to have well defined abs show like that, you have to have a extremely low body fat level and work out like crazy. Tons of cardio. Tons of ab exercises.
All abs and no brains....sounds like a real winner to me! (sarc)
Speaking of genes and women, wouldn't it be a greater sign of patriotism to settle down and have 5 or 6 kids rather than expose yourself to danger in the military? Darwin was critical of the role of standing armies in regards to exposing generally the healthiest males to death rather than being able to reproduce -- he'd certainly have had a lot of criticism for letting such women sign up for the military.