bit by bit vs little by little

bit by bit and little by little both are adverbs.

Word	Noun	Adjective	Verb	Adverb
bit by bit	No	No	No	Yes
little by little	No	No	No	Yes

As adverbs, bit by bit and little by little are synonyms defined as:

Other synonyms of bit by bit include in stages, piecemeal.

bit by bit (adverb)	little by little (adverb)
in a gradual manner	a little bit at a time
a little bit at a time	by a short distance