small letter vs grapheme

small letter and grapheme both are nouns.

Word	Noun	Adjective	Verb	Adverb
small letter	Yes	No	No	No
grapheme	Yes	No	No	No

As nouns, grapheme is a hypernym of small letter; that is, grapheme is a word with a broader meaning than small letter:

small letter: the characters that were once kept in bottom half of a compositor's type case
grapheme: a written symbol that is used to represent speech

Other hypernyms of small letter include character, graph, graphic symbol.

small letter (noun)	grapheme (noun)
the characters that were once kept in bottom half of a compositor's type case	a written symbol that is used to represent speech