trim back vs decrease

trim back is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
trim back	No	No	Yes	No
decrease	Yes	No	Yes	No

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

Other hypernyms of trim back include lessen, minify.

trim back (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

trim back (verb)	decrease (verb)
cut down on; make a reduction in	decrease in size, extent, or range
	make become smaller