Is cardiomegaly reversible?

Sometimes: This needs to be confirmed- sometimes the heart just appears large on chest xray, and radiologist calls "cardiomegaly" need definitive chamber size assessment with echocardiogram (most commonly) or with a cardiac ct or cardiac MRI (not commonly first line tests). Depending on the cause of the heart enlargement, there may be treatments which can improve heart function/reduce chamber enlargement.

Depends on the cause: Cardiomegaly is an enlarged heart. This could be relatively normal, as in an athlete, or due to a more serious problem. It needs further definition, usually with an echocardiogram, to determine the cause. Cardiomegaly is usually a secondary issue. Find the primary issue causing the cardiomegaly and fix that, if you can. This needs a doctor visit.