What Does Heart Enlargement Mean In A Baby?

I just took my 6 month old daughter to the ER for cough and congestion. ER doctor swabbed her nose and did an X-ray to check for the Flu virus. Everything was fine but he was a little concerned with the size of her heart. He said it looked enlarged and suggested I follow up with her pediatrition. What does enlarged hear mean for a 6 month old?