after-change-bytes.el

(defvar-local bb--region-before-change nil)

(defun bb--before-change (beg end)
  (setq-local bb--region-before-change
              (cons beg (buffer-substring-no-properties beg end))))

(defun bb--after-change (beg end len)
  (let* ((pre-beg (car bb--region-before-change))
         (pre-str (cdr bb--region-before-change))
         (nbytes (string-bytes (substring pre-str (- pre-beg beg) len))))
    (let ((new (cons pre-beg
                     (concat (substring pre-str 0 (- beg pre-beg))
                             (buffer-substring-no-properties beg end)
                             (substring pre-str (+ len (- beg pre-beg)))))))
      (message "# bytes before change: %S" nbytes)
      (setq-local bb--region-before-change new))))

(defun bb--test ()
  (interactive)
  (add-hook 'before-change-functions 'bb--before-change nil t)
  (add-hook 'after-change-functions 'bb--after-change nil t))