fully grown vs big

fully grown and big both are adjectives.

Word	Noun	Adjective	Verb	Adverb
fully grown	No	Yes	No	No
big	No	Yes	No	Yes

As adjectives, fully grown and big are synonyms defined as:

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

fully grown (adjective)	big (adjective)
(of animals) fully developed	above average in size or number or quantity or magnitude or extent
	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