step down vs decrease

step down is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
step down	No	No	Yes	No
decrease	Yes	No	Yes	No

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

Other hypernyms of step down include lessen, minify.

step down (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

step down (verb)	decrease (verb)
reduce the level or intensity or size or scope of	decrease in size, extent, or range
give up or retire from a position	make become smaller