heaviness vs heftiness

heaviness and heftiness both are nouns.

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

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

Other hyponyms of heaviness include heft, massiveness, ponderosity, ponderousness, preponderance.

heaviness (noun)	heftiness (noun)
the property of being comparatively great in weight	possessing muscular strength
used of a line or mark	the property of being large in mass
unwelcome burdensome difficulty
an oppressive quality that is laborious and solemn and lacks grace or fluency
persisting sadness