adult vs big

adult is a noun but big is not a noun.

Word	Noun	Adjective	Verb	Adverb
adult	Yes	Yes	No	No
big	No	Yes	No	Yes

As adjectives, adult and big are synonyms defined as:

Other synonyms of adult include full-grown, fully grown, grown, grownup.

adult (noun)	big (noun)
any mature animal
a fully developed person from maturity onward

adult (adjective)	big (adjective)
(of animals) fully developed	above average in size or number or quantity or magnitude or extent
designed to arouse lust	conspicuous in position or importance
	given or giving freely
	generous and understanding and tolerant
	marked by intense physical force
	significant
	prodigious
	serious or severe
	loud and firm
	(of animals) fully developed
	in an advanced stage of pregnancy
	feeling self-importance
	exhibiting self-importance