That's an interesting question. First of all, the original size of human eyes is determined by gene. That's why some people have big eyes and some people don't. During the infant and preschool period, the size of eye ball will change. The diameter of human eye ball will be 2.3 to 2.5cm. It is the same between adults and school kids. Therefore, human eye ball keeps growing bigger before 6 to 8. And then the growth stops. In general sense, the size of eyes is not the size of eye ball but the size of palpebral fissure, the crevice between eyelids. You see, when people want to enlarge their eyes, they often go to double eyelids. It is just enlarger the palpebral fissure. The size of palpebral fissure changes with age. As children grow up, their face will grow as well as other parts of their bodies. Thus, the palpebral fissure will be bigger and so are eyes. The growth of face will stop around teenage. And in the later years of life, the skin loses flexibility and becomes sagging, the palpebral fissure will be smaller and so are eyes. People may think that babies have bigger eye than adults. It is because babies have tiny little face in which the eyes are at the position of about the upper half of the face. While getting older, people's face becomes bigger, and the eyes are at the position of the upper one third of the face. That's why compared with adults, babies have bigger eyes.
