thin vs trim back

thin is an adjective but trim back is not an adjective.

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

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

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

thin (adjective)	trim back (adjective)
of relatively small extent from one surface to the opposite or in cross section
relatively thin in consistency or low in density; not viscous
lacking excess flesh
(of sound) lacking resonance or volume
not dense
very narrow
lacking spirit or sincere effort
lacking substance or significance

thin (verb)	trim back (verb)
take off weight	cut down on; make a reduction in
lose thickness; become thin or thinner
make thin or thinner
lessen the strength or flavor of a solution or mixture

thin (adverb)	trim back (adverb)
without viscosity