Umm, very? People alive in the 1950s would find such a person pretty bizarre, as it's the kind of physique that literally couldn't be created prior to the invention of anabolic steroids in the 1930s and their popularization in bodybuilding circles in the 1960s and 1970s. It also takes an extremely specific diet to reach that level of musculature and another very specific diet to cut down to that level of body fat. Early 20th century strongmen and weightlifters were notably smaller and less cut.

To a great degree, this is a better question for an archaeologist. I am not an archaeologist, but I did read a few articles on the subject while staying at a Holiday Inn Express last night. However, I'm not sure archaeology can tell us all that much about body fat percentages or muscle mass. Textual and artistic evidence are probably more useful (or at least, it's what I'm familiar with), but they inform us more about diet and forms of exercise than body type.

People have a lot of misconceptions about the physiques of medieval people. They were not pygmies; the average height of a northern European man throughout most of the period was around 5'8", which is only an inch or two shorter than the average modern Briton or American. They were not consistently half-starved; the average medieval farmer ate a nutritious, if bland, diet. But it was a diet that was heavy in carbohydrates, especially gruel and weak beer, and relatively light in protein, which is good for keeping you on your feet during hours and hours of heavy labor, but not ideal for building a hugely muscular or low-fat body. Likewise, when they had strenuous work to do almost every day, going on a long cutting cycle would not have been practical. The common body type of a farmer (that is, 90% or so of the population) would probably be most similar to a modern manual laborer: strong, fit for long and hard toil, but not tremendously muscular. They could probably walk the bodybuilder to death, however.

Aristocrats typically consumed more meat and did far less labor, so it may well be that they were different in build. They seem to have been very slightly taller, about on par with a modern person. Hunting from horseback, dancing, swimming, and other physical activities were very common diversions, but they're not really great ways to build huge amounts of muscle. We have evidence that those who pursued a military vocation (at some times and places, almost all able-bodied males) undertook more rigorous exercise, such as fencing, jousting, wrestling, and lifting stones, but there's also no indication that they were especially bulky. Certainly the art of the time depicts warriors, rich and poor, as relatively slender and athletic. Art from the Middle Ages was highly stylized, but in combination with other evidence, I think it's a safe bet that they looked more like early 20th century football and hockey players than Dwayne Johnson.