This is because the frame must match the pupil height of the human eye. The best optical area of a lens is its thinnest area, and a lens has only one optical center point, which is generally at the geometric center of the lens, the closer to the center, the thinner it is, and the closer to the edge, the thicker it is. This optical center point should be aligned with the pupil height of the person.
Once you choose a large frame, the center of the lens must be moved up to match the height of the pupil of the human eye. The thickest part of the bottom half will be included in the frame, and some will retain the thickest part at the edge, so that the overall glasses will become heavier. , or the thicker edges look more ugly, which greatly affects the wearing comfort and the stability of the glasses.
