big fish vs knocker

big fish and knocker both are nouns.

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

As nouns, knocker is a hyponym of big fish; that is, knocker is a word with a more specific, narrower meaning than big fish:

big fish: an important influential person
knocker: (Yiddish) a big shot who knows it and acts that way; a boastful immoderate person

Other hyponyms of big fish include supremo.

big fish (noun)	knocker (noun)
an important influential person	a device (usually metal and ornamental) attached by a hinge to a door
	either of two soft fleshy milk-secreting glandular organs on the chest of a woman
	one who disparages or belittles the worth of something
	a person who knocks (as seeking to gain admittance)
	(Yiddish) a big shot who knows it and acts that way; a boastful immoderate person