The answer
Coptic art concentrates on the head more than on the body, and therefore at times, it may seem that the body is not in proportion with the head. Also, Coptic art focuses on the eyes, which are always portrayed larger than any other aspect of the head. The significance of the eyes are that they are watchful and concentrating on the vision to wards heaven. Hence, in art we focus on the eyes more than the head, and the head more than the body. There are many books and articles available concerning Coptic art, in more detail.
H.G. Bishop Moussa