People are a diverse collection of different genetics and body types. check out
this pictorial (warning: athletes in underwear) featuring the body types of many different athletes. You will see that even among those in the same field of athletics, there is a variety of body shapes and sizes.
Sure, every person has their own stereotypes of what a soldier "should" look like, based on historical depictions of soldiers in media, their own experiences, and what have you. But as the linked pictorial shows, people do come in all shapes and sizes, and perhaps we as a media culture should start accepting that different body shapes can still represent strength.