Read it again. There is a scene that shows his childhood confession in two points of view. He tells her, "ill marry you when I'm an adult"(taller). She takes it as, "ill marry you when I'm an adult (older). The schtik of this manga is that he is insecure about his height, while she likes him regardless of it.
This is one of the worst cases I've seen of the Japanese trope of being cold and catching a cold. Herp derp, cold shower, caught a cold. That's just bad.