The relationship is that anemias like hereditary elliptocytosis, hereditary spherocytosis, sickle cell disease and thalassemia can all cause splenomegaly. Splenomegaly isn't itself a disease or anything like that. It's a symptom. Certain infections can also cause splenomegaly. So can cancer and so can other disorders. In fact, there are 26 disorders that can enlarge the spleen.
And when for any reason the spleen removes too many blood cells and platelets from the bloodstream for destruction, a variety of problems may develop. These problems include anemia as a result of too few red blood cells. This can happen if the spleen is enlarged or diseased from some other disorder. One of the spleen's functions is to remove old, worn out red blood cells from the blood. Inherited conditions such as thalessemia, a type of anemia, cause a more rapid cell death. The spleen can become enlarged from removing so many extra blood cells and, in turn, create a situation of iron-deficiency anemia.
So the spleen-anemia relationship can work both ways. One can cause the other and vice versa.
I'm a nurse.