Pardon the rub-rub orgy in the background. Cirno grew this little one a few days ago and she's actually grown in size a bit. You can also see the opposite mix of a Marisa/Cirno on the floor there, too.
Huh, so they actually increase in size now? In the old version of the game, as far as I could tell, all the yus were fixed at whatever size they began at (so babies were babies forever, and such).