small capital vs capital letter

small capital and capital letter both are nouns.

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

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

small capital: 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 capital include capital, majuscule, upper-case letter, uppercase.

small capital (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