When you format text as small caps, Illustrator automatically uses the small-cap characters designed as part of the font, if available. Otherwise, Illustrator synthesizes the small caps using scaled-down versions of the regular capital letters.
To specify the size for synthesized small caps, choose File > Document Setup, and choose Type from the pop‑up menu at the top of the dialog box. For Small Caps, type a percentage of the original font size for text to be formatted as small caps. (The default value is 70%.)