It's been clearly hinted that Nezuko's size varies depending on her mood. Since she's undead-ish, she no longer ages and is stuck at 12 (and most of the action takes place 2/3 years after her transformation).

Re-watch some of the episodes of season 1, she clearly grows to fight and shrink to fit in the box. Demons have some pretty extreme abilities and control over their bodies, and this is her way to do it.

Well put me down as scared AND horny