She is a Primera.
I just understood this too, but yes, she's a Primera. Essentially, they have longer lifespans that most other humans and they have great magical power. However, the price is that whenever she uses magic it take some time off her life. So, if she doesn't use magic she'd just age really slowly (hence why she looks tiny at 7) but when she does she will grow equal to the amount of time that it cost her to cast whatever magic used.
And since the magic she has cast only costs her "days", there hasn't been much growth.
Why is she still so small. Comparing her to the others.. she looks kind of weird- IM SORRY!!