small letter vs graphic symbol

small letter and graphic symbol both are nouns.

Word	Noun	Adjective	Verb	Adverb
small letter	Yes	No	No	No
graphic symbol	Yes	No	No	No

As nouns, graphic symbol is a hypernym of small letter; that is, graphic symbol 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
graphic symbol: a written symbol that is used to represent speech

Other hypernyms of small letter include character, graph, grapheme.

small letter (noun)	graphic symbol (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