small cap vs capital letter

small cap and capital letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
small cap	Yes	No	No	No
capital letter	Yes	No	No	No

As nouns, capital letter is a hypernym of small cap; that is, capital 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
capital 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, majuscule, upper-case letter, uppercase.

small cap (noun)	capital 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