shrink vs shrink back

shrink is a noun but shrink back is not a noun.

Word	Noun	Adjective	Verb	Adverb
shrink	Yes	No	Yes	No
shrink back	No	No	Yes	No

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

Other hyponyms of shrink include retract.

shrink (noun)	shrink back (noun)
a physician who specializes in psychiatry

shrink (verb)	shrink back (verb)
become smaller or draw together	pull away from a source of disgust or fear
reduce in size; reduce physically
wither, as with a loss of moisture
decrease in size, range, or extent
draw back, as with fear or pain