It's the smile that sells it to me. Goddamn adorable. Gotta give her kisses to make it all better.

It's likely going to be revealed that the thing that caused her height to be what it is was fairly recent (the only time a virus in Resident Evil worked slowly was the T-Veronica virus, which was slow enough that Alexia had to go into cryostasis for years).

Or she just doesn't want to screw up the castle architecture. Either way really.