I like to think that, the first time Dazai met Chuuya, he made fun of Chuuya's height without realizing from his position on the ground that he was only five centimeters taller. And when he did find out, he went "ah. well, in too deep to stop now, ima just go with it" and kept up the short jokes, despite the fact that they were practically the exact. same. height.