small beer vs bagatelle

small beer and bagatelle both are nouns.

Word	Noun	Adjective	Verb	Adverb
small beer	Yes	No	No	No
bagatelle	Yes	No	No	No

As nouns, bagatelle is a hyponym of small beer; that is, bagatelle is a word with a more specific, narrower meaning than small beer:

Other hyponyms of small beer include fluff, frippery, frivolity.

small beer (noun)	bagatelle (noun)
something of small importance	a table game in which short cues are used to knock balls into holes that are guarded by wooden pegs; penalties are incurred if the pegs are knocked over
	something of little value or significance
	a light piece of music for piano