in the main vs principally

in the main and principally both are adverbs.

Word	Noun	Adjective	Verb	Adverb
in the main	No	No	No	Yes
principally	No	No	No	Yes

As adverbs, in the main and principally are synonyms defined as:

Other synonyms of in the main include chiefly, mainly, primarily.

in the main (adverb)	principally (adverb)
without distinction of one from others	for the most part
for the most part