dwindle down vs decrease

dwindle down is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
dwindle down	No	No	Yes	No
decrease	Yes	No	Yes	No

As verbs, decrease is a hypernym of dwindle down; that is, decrease is a word with a broader meaning than dwindle down:

Other hypernyms of dwindle down include diminish, fall, lessen.

dwindle down (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

dwindle down (verb)	decrease (verb)
become smaller or lose substance	decrease in size, extent, or range
	make become smaller