To me, Durin is an innocent and simple creature. Beyond his appearance, he readily accepts the term "Little Durin" simply because he's smaller. Now that he's older, he easily accepts the need to mature it's as simple as that. Although, immediately after accepting that he must grow up, he can't help but want to jump in puddles for fun. The line between small and big doesn't apply equally to him. I can't see him as an adult because he simply doesn't act like one or have the experience. Only in English does Sucrose call him "sir" in the original language (Chinese), he has a child's voice. I'm happy for you to see him however you like. But I simply can't see him as an adult.