Cardiomegaly is quite common in thals and its incidence increases with lower hemoglobin levels. It is also more common with the more transfusions given. It is very likely that your lower Hb levels has affected the cardiomegaly. It is not likely that the cardiomegaly has caused the lower Hb level. The lower Hb causes your heart to work harder which would also explain the faster heart beat and tiredness.
Cardiomegaly in thals does not necessarily affect overall heart function, although it can be a sign of a more serious problem. As always, maintaining an adequate Hb level and low ferritin levels are keys to maintaining good heart function. There may still be some cardiomegaly as patients age because of the frequent transfusions, and much research is being done in this area as the problems of older thals become more apparent as the thal population ages, which in itself is a great development. There is also evidence that defriprone (L1, Kelfer, Ferriprox) chelation therapy over the long term, is associated with greater improvement in cardiac function than desferal therapy.