narrowness vs narrow margin

narrowness and narrow margin both are nouns.

Word	Noun	Adjective	Verb	Adverb
narrowness	Yes	No	No	No
narrow margin	Yes	No	No	No

As nouns, narrowness and narrow margin are synonyms defined as:

Other synonyms of narrowness include slimness.

narrowness (noun)	narrow margin (noun)
the property of being narrow; having little width	a small margin
a small margin
an inclination to criticize opposing opinions or shocking behavior
a restriction of range or scope