While they might not actually be the stereotype as you said, they were still a class above your average raider or peasant soldier employed by many other nations, both physically and mentally.

To be a Legionary, you had to meet a very demanding physical standard, with some documents stating that a soldier could not have a protruding gut, and would have to be over 5'7" (in modern measurements). These standards may have been "fudged" a little depending on the province in which the Legion in question was raised (They may lower the height requirement if nobody is tall enough, for example), but they were still designed to only recruit the most physically capable men of the area. They had to be capable of carrying their arms and armor long distances in forced marches (Only the officers and wealthy soldiers could afford to purchase horses or mules to carry their gear for them). They needed to work in close confines of a tight-knit phalanx, carrying a shield in one hand and their short but heavy gladius in the other. They needed to be strong enough to pierce armor with their gladius and be capable of throwing a pilum (a throwing spear designed to break or bend upon impact with a hard surface, so it couldn't be thrown back), so they required a very strong upper body.

Point is, while they may not have had the build of Adonis, they still were very physically fit and strong, and would have bodies to match that.