small letter vs graph

small letter and graph both are nouns.

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

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

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

small letter (noun)	graph (noun)
the characters that were once kept in bottom half of a compositor's type case	a visual representation of the relations between certain quantities, represented as points, plotted with reference to a set of axes
	a visual display of information
	a written symbol that is used to represent speech

small letter (verb)	graph (verb)
	represent by means of a graph
	plot upon a graph