Strength comes in all shapes and sizes, forms and figures.
I don't believe "skinny" equals strong. I don't believe "fat" equals unhealthy. I also don't believe a body should be the only representation of strength, but the physical presence of strength can (and often does) say a lot about a person's ability to test themselves.
Strong women need to be represented, because we've spent long enough being pigeon-holed as delicate, small and ill equipped.