Oysters are protantrous hermaphrodites, meaning that htey start off life as male, then most will change to female. Oysters reach puberty and have the ability to reproduce after about one year. After that point, almost all of them are male, but they begin to change to female at this time. Although other species of oysters can change back and forth between male and female, Crassostrea virginica cannot. Therefore, one can make an educated guess as to the gender of an oyster depending on size, but it is important to realize that you may not be correct in your guess.
Larger oysters are probably females while the small ones are males.
Maybe it has something to do with more energy is needed to produce the larger eggs than sperm. After three months, the spat is about the size of a dime. After one year it is sexually mature and can spawn. Remember, growth is salinity and temperature dependent. Oysters can live up to 10-12 years, although it is uncommon as they are usually harvested, caught or eaten by predators before they reach that age.