strength vs heftiness

strength and heftiness both are nouns.

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

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

strength (noun)	heftiness (noun)
the property of being physically or mentally strong	possessing muscular strength
an asset of special worth or utility	the property of being large in mass
permanence by virtue of the power to resist stress or force
the amount of energy transmitted (as by acoustic or electromagnetic radiation)
capability in terms of personnel and materiel that affect the capacity to fight a war
physical energy or intensity
capacity to produce strong physiological or chemical effects
the power to induce the taking of a course of action or the embracing of a point of view by means of argument or entreaty
the condition of financial success