thin out vs cut back

thin out and cut back both are verbs.

Word	Noun	Adjective	Verb	Adverb
thin out	No	No	Yes	No
cut back	No	No	Yes	No

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

Other hypernyms of thin out include bring down, cut, cut down, reduce, trim, trim back, trim down.

As verbs, cut back is a hyponym of thin out; that is, cut back is a word with a more specific, narrower meaning than thin out:

Other hyponyms of thin out include clip, crop, dress, lop, prune, snip, trim.

thin out (verb)	cut back (verb)
make sparse	place restrictions on
become sparser	cut down on; make a reduction in
lessen the strength or flavor of a solution or mixture	cultivate, tend, and cut back the growth of
	return in time