Overview
Miyagawa Atsushi is an 11-year-old boy who is 180 centimeters tall. Since he has a grown-up looking build but acts his age, he has always been running into troubles. On the other hand, his 17-year-old elder sister Atsumi is only 137 centimeters tall. Even though she has a childish looking build, Atsumi takes good care of Atsushi.
(Source: MyAnimeList
It looks fast and easy.
However, the third season was ruined by the shift. I don't know why this was done, because the studio seems to have remained the same.