When Dean is changed to a much physically smaller teenager, he disappears out of his clothes. as a teenager, his new clothes fit him perfectly. when he changes back to his adult size, he is still wearing the youth clothes but they fit his bigger frame fine.
On further examination of the clothes, it's obvious that they're baggy and a little too large for teenage Dean, and when he's restored to his normal age, the sleeves are a little short.
When Dean is turned into a teenager, it is shown that the Mark of Cain disappears from his arm. In a later episode it is revealed that, if the mark were to be removed without being transferred to another person, the darkness would be released. Given this information, the darkness should have been released due to Dean no longer having the mark.