Why does this API keep increasing the font size? With a selection, try the test command.

Alternatively, what's the best way to query the font-size at point and re-apply using face property?

font-info

[ OPENED-NAME FULL-NAME SIZE HEIGHT BASELINE-OFFSET RELATIVE-COMPOSE
    DEFAULT-ASCENT MAX-WIDTH ASCENT DESCENT SPACE-WIDTH AVERAGE-WIDTH
    FILENAME CAPABILITY ]

(defun test(start end)
  (interactive "r")
  (let* ((size (* 10 (aref (font-info (font-at start)) 3))))
    (put-text-property start end 'face
      (list :inherit 'unspecified :height size))))