AABB-pattern reduplication of the antonym pair
大 /
小. The doubling intensifies and pluralizes — 'all the various sizes, none excepted'. The same template generates
高高低低 (gāogāo-dīdī, 'all heights'),
男男女女 (men and women alike),
老老少少 (old and young),
来来往往 (coming and going).
