Macrocytosis is a term used to describe red blood cells that are larger than normal. Anemia is when you have low numbers of properly functioning red blood cells in your body. Macrocytic anemia, then, is a condition in which your body has overly large red blood cells and not enough normal red blood cells. Different types of macrocytic anemia can be classified depending on what’s causing it. Most often, macrocytic anemias are caused by a lack of vitamin B-12 and folate. Macrocytic anemia can also signal an underlying condition.