Whether or not there is a problem depends on the underlying condition. Some individuals have a normally large heart, and non-pathological enlarged heart can be the result of a well-conditioned athlete.
Pathological enlarged heart can be the result of an overworked heart pumping against a high resistence of constricted vessels, the heart walls can be enlarged due to heart muscle disease, etc. or a prior heart attack or ischemic (lack of blood/oxygen due to vessel occlusion. etc.) that impair heart muscle contractions.
Whether or not aerobic exercise is beneficial may depend on the underlying cause. Did the doctor state an opinion regarding cause?
Hi. I think you should try posting this question in the adult heart expert forum.