heftiness vs brawn

heftiness and brawn both are nouns.

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

As nouns, heftiness and brawn are synonyms defined as:

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

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