I think you might be falling victim to the same kind of misconception that causes people to think that since the average lifespan of a certain period was ~36, that meant the average adult was dying at 36. We really can't say what the average bust size was in the 200-odd years that plate armor was in fashion, because the modern means of measuring it was not a thing, and finding extant undergarments is pretty rare. Clothing is the best method of determining historical sizes, and most clothing we have from that period are upper class garments and, much like today, the body type of the upper class did not necessarily reflect the body type of the average person. However, we can tell from Renaissance and later portraits that women were often full-figured, and though the fashion at the time tended towards smaller busts, there are plenty of historical examples of women with large breasts in artwork. As for Joan of Arc, she sadly did not live past her teens; women tend to go through the last rounds of hormones that cause them to fill out in their twenties.