I'm seeing this sentiment a lot here that she's finally looking her age, when there's nothing to indicate that's what Erwärmen/Anwachs was aiming for. Erwärmen only said that her vessel was too small to contain the Book of Mestionora, not that she was too small for her age. For all we know if she had already looked her age (noble age or true age) then Erwärmen may have still had her grow a bit more.
All we really know is that she now has a large enough "vessel" (presumably a large enough capacity for mana/schtappe) for the Book of Mestionora.
Personally when I first read that side story I thought the smirk was more because she gleamed some information from Veronica's wailing. I was pretty focused on trying to figure out what sort of information Veronica might have encoded or if it was just painting a target on Rozemyne and Ferdinand for Georgine. Didn't realize it until much later that that was just schadenfreude on her part, so I wouldn't really blame someone for missing that on first reading either.
With how much you can read into some of the interactions and events, it can be easy to overanalyze a simple smirk.