lessening vs shrinkage

lessening and shrinkage both are nouns.

Word	Noun	Adjective	Verb	Adverb
lessening	Yes	No	No	No
shrinkage	Yes	No	No	No

As nouns, shrinkage is a hyponym of lessening; that is, shrinkage is a word with a more specific, narrower meaning than lessening:

Other hyponyms of lessening include shrinking, casualty, sinking, attrition, rarefaction, dwindling, dwindling away, waning.

lessening (noun)	shrinkage (noun)
a change downward	the act of stealing goods that are on display in a store
	the amount by which something shrinks
	process or result of becoming less or smaller