big vs full-grown

big and full-grown both are adjectives.

Word	Noun	Adjective	Verb	Adverb
big	No	Yes	No	Yes
full-grown	No	Yes	No	No

As adjectives, big and full-grown are synonyms defined as:

Other synonyms of big include adult, fully grown, grown, grownup.

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