big businessman vs magnate

big businessman and magnate both are nouns.

Word	Noun	Adjective	Verb	Adverb
big businessman	Yes	No	No	No
magnate	Yes	No	No	No

As nouns, big businessman and magnate are synonyms defined as:

Other synonyms of big businessman include baron, business leader, king, mogul, power, top executive, tycoon.

big businessman (noun)	magnate (noun)
a very wealthy or powerful businessman	a very wealthy or powerful businessman