Hello supreetk87,
A mildly enlarged heart on chest x-ray can mean many, many things.
In fact, it is impossible to say what could be the cause of this finding without knowing more of your father's medical history. Long-standing high blood pressure is a common cause of an enlarged heart.
Honestly, you father should evaluated by a physician who can examine him to determine the severity and meaning of this test.
To your health,
Neil MD