An enlarged heart usually refers to an enlargement of the left ventricle (pumping chamber). The enlargement (dilated type) happens when the heart is overworked and pumping against a high resistence of constricted vessels, increased volume of fluids, etc.
If and when the heart reaches a detrimental dimension the heart walls lose its flexibility, and there is a reduction of cardiac output and the heart fails to supply adequately blood/oxygen to the system.
Medication that reduces the heart's workload sometimes enables a reduction of heart size and normal functioning accomplished with medication that dilates vessels, reduce fluids, strengthen contractions, reduce heart rate, etc.
What are the medical terms? What I have submitted post is the most common.