dwindling vs drop-off

dwindling and drop-off both are nouns.

Word	Noun	Adjective	Verb	Adverb
dwindling	Yes	Yes	No	No
drop-off	Yes	No	No	No

As nouns, drop-off is a hypernym of dwindling; that is, drop-off is a word with a broader meaning than dwindling:

Other hypernyms of dwindling include decrease, lessening.

dwindling (noun)	drop-off (noun)
a becoming gradually less	a change downward
	a steep high face of rock
	a noticeable deterioration in performance or quality

dwindling (adjective)	drop-off (adjective)
gradually decreasing until little remains