text: The difference in relative size of one body part with respect to another (generally speaking, with respect to body size) is called allometry (from allos "different" + metro "measurement"). And from a developmental viewpoint, it is determined by differences in growth rate between those parts, or in the onset or offset of the growth period (collectively known as heterochronies). \nThose differences may be directly coded in the person's DNA (especially in the general developmental homeotic genes) but can also arise from interaction among tissues and chemical signals between cells during growth. And in the case of abnormal size, there can be problems of embryonic development, hormones or others involved.