Enlarged red blood cells. Conceptual coloured scanning electron micrograph (SEM) of enlarged red blood cells (erythrocytes). Macrocytosis is a condition in which red blood cells are larger than they should be. While it isn't a condition of its own, macrocytosis is a sign of an underlying health condition. Common causes of macrocytosis include: vitamin B-12 deficiency, folate deficiency, liver disease, alcoholism, hypothyroidism, a side effect of certain medications, such as those used to treat cancer, seizures and autoimmune disorders, increased red blood cell production by the bone marrow to correct anemia, for example, after blood loss, and an underlying bone marrow cancer called myelodysplastic syndrome. Magnification x 2000 at 10cm wide.