they apparently had to conserve a weird notion of age difference=height difference

From what I've seen of the adaptations (mostly cover art, posters, the occasional clip), they all seem to follow the same trend: Davie is a boy, so he is short; Alan is an adult, so he is tall. It's very disappointing. The statue of them in Edinburgh does the same thing, too, from the photos I've seen. (Some wikipedia editor out there is as exercised about this topic as I am, for the wikipedia page for the statue firmly points out that the statue got their heights wrong.)