cardiomegaly vs enlarged heart

cardiomegaly and enlarged heart both are nouns.

Word	Noun	Adjective	Verb	Adverb
cardiomegaly	Yes	No	No	No
enlarged heart	Yes	No	No	No

As nouns, cardiomegaly and enlarged heart are synonyms defined as:

Other synonyms of cardiomegaly include megacardia, megalocardia.

cardiomegaly (noun)	enlarged heart (noun)
an abnormal enlargement of the heart	an abnormal enlargement of the heart