It's because she's a primera. They perform magic by burning off their life force. If she doesn't, her growth is basically at stalemate. You can see her progress now because of her teeth falling out and her hair growing. She basically went to perform healing to subtly release it so it would look like a natural growth because it take less power than attack spells. (⌒▽⌒)
I’m not 100% about this but I think primeras have 2 life forces, so it’s like aging 2x slower than normal. If you half her current age, her size makes more sense bc of this slower aging. Using her life force for magic and stuff ages her faster but not using it means she’ll age just rly slowly which is abnormal
Bro it's already chapter 80++ and yet she still looked the same from chapter 1