A four-month-old baby with one large eye and one small eye may be caused by physiological reasons, inconsistency in the strength of the levator muscles of both eyes, and asymmetry in the development of both eyelids or eyeballs. 1. Physiological reasons: it is common for four-month-old babies to have one eye big and one eye small, mostly because the baby’s eyes are not yet fully developed, which is a normal physiological phenomenon. 2. Inconsistency in the strength of the levator muscles of both eyes: when the strength of the levator muscles of both eyes of a four-month-old baby is inconsistent, it will easily lead to different degrees of ptosis, and one eye is big and the other eye is small. 3. Asymmetrical development of both eyelids or eyeballs: If a four-month-old baby has asymmetrical development of both eyelids or eyeballs, such as a small eyeball in one eye, it may lead to the condition of one eye being big and the other small. Four-month-old babies with one eye big and one eye small if it is more obvious, leading to the baby’s vision abnormalities, parents should take the baby to the hospital in time to consult the doctor, to clarify the cause of the disease and then actively treat.