The girl, Risa Koizumi, is  tall—much taller than the average Japanese girl. The boy, Atsushi Ōtani, is —way below the height of the average Japanese boy.
In this context, Risa Koizumi's exceptional height sets her apart from the typical Japanese girl, while Atsushi Ōtani's short stature makes him stand out as being below average for a Japanese boy.