boxer vs heavyweight

boxer and heavyweight both are nouns.

Word	Noun	Adjective	Verb	Adverb
boxer	Yes	No	No	No
heavyweight	Yes	No	No	No

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

boxer (noun)	heavyweight (noun)
a breed of stocky medium-sized short-haired dog with a brindled coat and square-jawed muzzle developed in Germany	A weight class division in combat sports, often the heaviest.
someone who fights with his fists for sport	a person of exceptional importance and reputation
a workman employed to pack things into containers	a professional boxer who weighs more than 190 pounds
	a wrestler who weighs more than 214 pounds
	a very large person; impressive in size or qualities
	an amateur boxer who weighs no more than 201 pounds