to a great extent vs heavily

to a great extent and heavily both are adverbs.

Word	Noun	Adjective	Verb	Adverb
to a great extent	No	No	No	Yes
heavily	No	No	No	Yes

As adverbs, to a great extent and heavily are synonyms defined as:

to a great extent (adverb)	heavily (adverb)
to a considerable degree	to a considerable degree
	indulging excessively
	slowly as if burdened by much weight
	in a manner designed for heavy duty
	in a heavy-footed manner
	with great force
	in a labored manner