dwindling away vs decrease

dwindling away and decrease both are nouns.

Word	Noun	Adjective	Verb	Adverb
dwindling away	Yes	No	No	No
decrease	Yes	No	Yes	No

As nouns, decrease is a hypernym of dwindling away; that is, decrease is a word with a broader meaning than dwindling away:

Other hypernyms of dwindling away include drop-off, lessening.

dwindling away (noun)	decrease (noun)
a becoming gradually less	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

dwindling away (verb)	decrease (verb)
	decrease in size, extent, or range
	make become smaller