mostly vs by and large

mostly and by and large both are adverbs.

Word	Noun	Adjective	Verb	Adverb
mostly	No	No	No	Yes
by and large	No	No	No	Yes

As adverbs, mostly and by and large are synonyms defined as:

Other synonyms of mostly include generally, more often than not.

mostly (adverb)	by and large (adverb)
in large part; mainly or chiefly	usually; as a rule
usually; as a rule