In the latter case, the disease creates blockages in the hearts blood supply, leading to tissue death which causes other areas of the heart to work harder, causing the heart to expand in size.Other possible causes include:

Heart valve disease
Cardiomyopathy (disease of the heart muscle)
Pulmonary hypertension
Pericardial effusion (fluid around the heart)
Thyroid Disorders
Hemochromatosis (excessive iron in the blood)
Amyloidosis
Chagas disease, an important cause of cardiomegaly in Latin America
Viral infection of the heart
Pregnancy, with enlarged heart developing around the time of delivery (peripartum cardiomyopathy)
Kidney disease requiring dialysis
Alcohol or cocaine abuse
HIV infection
Diabetes

Mechanism
Cardiomegaly is a condition affecting the cardiovascular system, specifically the heart. This condition is strongly associated with congestive heart failure. Within the heart, the working fibers of the myocardial tissue increase in size. As the heart works harder the actin and myosin filaments experience less overlap which increases the size of the myocardial fibers. If there is less overlap of the protein filaments within the sarcomeres of the muscle fibers, they will not be able to effectively pull on one another. If the heart tissue gets too big and stretches too far, then those filaments cannot effectively pull on one another to shorten the muscle fibers, impacting the hearts sliding filament mechanism. If fibers cannot shorten properly and the heart cannot contract properly, then blood cannot be effectively pumped to the lungs to be re-oxygenated or to the body to deliver oxygen to the working tissues of the body.A person with an enlarged heart is more susceptible to forming blood clots in the lining of their heart.
LVADs are often implanted while a patient waits for a heart transplant or, if the patient is not a heart transplant candidate, as a long-term treatment for heart failure. Heart transplant: If medications cant control the symptoms, a heart transplant is often a final option.Cardiomegaly can progress and certain complications are common:

Heart failure: One of the most serious types of enlarged heart, an enlarged left ventricle, increases the risk of heart failure. In heart failure, the heart muscle weakens, and the ventricles stretch (dilate) to the point that the heart cant pump blood efficiently throughout the body. Blood clots: Having an enlarged heart may make one more susceptible to forming blood clots in the lining of the heart. If clots enter the bloodstream, they can block blood flow to vital organs, even causing a heart attack or stroke. Clots that develop on the right side of the heart may travel to the lungs, a dangerous condition called a pulmonary embolism. Heart murmur: For people who have an enlarged heart, two of the hearts four valves — the mitral and tricuspid valves — may not close properly because they become dilated, leading to a backflow of blood. This flow creates sounds called heart murmurs. The exact mortality rate for people with cardiomegaly is unknown. However, many people live for a very long time with an enlarged heart and, if detected early, treatment can help improve the condition and prolong their lives.