An 'enlarged heart' is the term used to describe a heart with an excessively thickened muscular wall or, alternatively, an over-stretched heart with dilated chambers.
Both conditions impair the efficiency of the heart's pumping action and can be associated with heart failure.
A thickened cardiac muscle nearly always results from prolonged and uncontrolled high blood pressure. Occasionally it's due to a tight exit valve (the aortic valve), and very rarely it is caused by an inherited disease of heart muscle.
An over-stretched heart is the result of cardiac muscle damage of any sort, most often due to coronary heart disease (furring-up of arteries supplying blood to the heart itself).