Myopia, or nearsightedness, happens when the eye grows a little too long from front to back. That length is called the axial length, and it is the real measure of myopia. As a child grows, the eye keeps lengthening, which is why a child's prescription can climb at every visit.
This matters for more than thicker glasses. A longer eye is associated with a higher lifetime risk of serious conditions later in life, including macular degeneration, retinal detachment, and glaucoma. The goal of myopia control is to keep both the prescription and the axial length as low as possible during the growing years.