When your lunula is larger than normal — about 50% of the nail — it could be a symptom of an endocrine disorder, infectious disease, cardiac issue, or congenital disorder (via ePainAssist).
Per a case study published in Cureus, enlarged lunulae can be associated with hyperthyroidism, leprosy, and scleroderma. Hyperthyroidism is a condition where the thyroid overproduces hormones, while scleroderma is an uncommon skin disease that leads to tightness and hardening of the skin, specifically the hands, feet, and face (via the Mayo Clinic). On the other hand, leprosy is a bacterial infection that causes discolored patches of skin and muscle weakness, according to the Centers for Disease Control and Prevention.
Additionally, the Cureus study notes that it could be trauma to the nail causing the deformity. Topical application of the steroid medicine hydrocortisone could also cause the half-moon on your nail to increase in size.