Let's be real, both girls reverting to normal size was bound to happen.
However, it looks like we might not get the "Normal Romance" ending after all. It all depends on whatever those two will talk about on their way home. My guess is that Chieri (accidentally?) reveals she wants to keep depending on him just like before. Added to the selfless desire of helping whoever needs a huge hand to reach she gained on the way, it could trigger gigantification once again, this time unlikely to be fixable due to the nature of her wishes. On top of this, Yumeji further fueling those feelings by asking to remain by her side as a helper isn't out of the question as well.
And that's how they end up being a unusual couple with a massive size difference.