big fish vs big deal

big fish and big deal both are nouns.

Word	Noun	Adjective	Verb	Adverb
big fish	Yes	No	No	No
big deal	Yes	No	No	No

As nouns, big fish and big deal are synonyms defined as:

Other synonyms of big fish include big cheese, big enchilada, big gun, big shot, big wheel, head honcho.

big fish (noun)	big deal (noun)
an important influential person	anything of great importance or consequence
	an important influential person