brawn vs heftiness

brawn and heftiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
brawn	Yes	No	No	No
heftiness	Yes	No	No	No

As nouns, brawn and heftiness are synonyms defined as:

Other synonyms of brawn include brawniness, muscle, muscularity, sinew.

brawn (noun)	heftiness (noun)
possessing muscular strength	possessing muscular strength
	the property of being large in mass