An enlarged heart
) comes from the heart having to pump harder in order to deliver blood to the rest of the body. Since the heart is a muscle it can enlarge like any of the body's other muscles that are subjected to an increased work load.
How bad it is depends on a lot of different things: are there any other preexisting cardiac conditions or vascular conditions? Its not always something that is necessarily bad or life threatening; it can be benign.
Doctor will have to run tests and labs to determine things like how much blood is actually being pumped out of the heart, etc.