big fish vs big enchilada

big fish and big enchilada both are nouns.

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

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

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

big fish (noun)	big enchilada (noun)
an important influential person	an important influential person