thin out vs decrease

thin out is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
thin out	No	No	Yes	No
decrease	Yes	No	Yes	No

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

Other hypernyms of thin out include diminish, fall, lessen.

thin out (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

thin out (verb)	decrease (verb)
make sparse	decrease in size, extent, or range
become sparser	make become smaller
lessen the strength or flavor of a solution or mixture