You don't know how or what just happened, but honestly, it's best not to question it. Looks like you got shrunken down by him, and he's just unaware of what is about to happen to you as he's about to take a step. After all, you're smaller than Harry, and you can tell he doesn't even know where you'll be at after this...

This is Version A, with you being the one shrunken down. The B version is where he's bigger than the skies.