I am not completely sure about the specifics, but heart enlargement is usually caused by the hearts inability to pump enough blood, thus the heart muscle produces more muscle tissue and enlarges to try and compensate for the lack of pumping volume.
I believe there are a few things that can cause it. If I remember correctly, a virus, drugs, and hereditary issues can cause it to name a few. I am sure there are more though.
As far as where to learn more, browse this site or do a google search for enlarged heart and you will have hours of reading material!!
frank