big fish vs big wheel

big fish and big wheel both are nouns.

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

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

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

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