I don't know how Sailor Moon transforms, but I think it's more subtle than the Hulk. It's not that a small body turns into a large one; the small body fades out (into astral? nothing?) while the dragon body fades in. It's surprisingly non-messy for such a massive size difference.
No idea what happens when a Great Dragon decides to shift from metahuman to dragon form while in a small enclosed space. They probably don't decide to do things like that.