giantess vs adult female

giantess and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
giantess	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of giantess; that is, adult female is a word with a broader meaning than giantess:

Other hypernyms of giantess include giant, woman.

giantess (noun)	adult female (noun)
a female giant	an adult female person (as opposed to a man)