big enchilada vs big fish

big enchilada and big fish both are nouns.

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

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

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

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