heavily vs to a great extent

heavily and to a great extent both are adverbs.

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

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

heavily (adverb)	to a great extent (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