Athletic heart syndrome i thought that enlargement of the heart is very bad, is it no?

In general yes, but: Sometimes, cardiac enlargement can reflect heart failure. In other cases, cardiomyopathy may develop, where the chambers may enlarge because of illness of the heart muscle. However, some trained athletes have enlarged hearts from prolonged training without any evidence of dysfunction or arrhythmia risk. This has been termed "athletic heart"--but telling the difference can sometimes be difficult.