Athetic in the modern sense of a dedicated athlete maybe. They have the athletic build of a modern person doing modern exercises. Nobody did those sort of excercise in the middle ages, they didn't develop any muscle aside from what they used in their day to day lives, which wasn't much. Even soldiers wouldn't develop muscle that way, except strictly where they needed it for swinging a sword.
In Ser Cambria's defense, there were bodybuilding (of a sort) athletes in the past in ancient Greece and Rome. Most people weren't toned then either, but there certainly were some--hence where their statues of the ideal human form came from.
So the muscular human form would not have been an alien concept to medieval people, who would have seen those statues around--a lot of the old temples and such hadn't fallen totally into ruin yet. I am equally sure that some people did in fact have that sort of build--just not many. And yes, there would have been people who went out of their way to have such a physique, if only for entertainment value (actors, minstrels, and the like who would have entertained crowds of people. Not as likely for women as for men--and at some point I do believe a pot belly was considered attractive, though not sure if that was in the Middle Ages or not).