small cap vs upper-case letter

small cap and upper-case letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
small cap	Yes	No	No	No
upper-case letter	Yes	No	No	No

As nouns, upper-case letter is a hypernym of small cap; that is, upper-case letter is a word with a broader meaning than small cap:

small cap: a character having the form of an upper-case letter but the same height as lower-case letters
upper-case letter: one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis

Other hypernyms of small cap include capital, capital letter, majuscule, uppercase.

small cap (noun)	upper-case letter (noun)
a character having the form of an upper-case letter but the same height as lower-case letters	one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis
a corporation with a small capitalization