The author should have made longer time skips. She doesn't seem like nine years old at all. Maybe I could believe the 12, with how tough a life she has lived. Of course she has all that magic and other fantasy boosts helping her, allowing her to beat adults, so there is that. Though this is not 100% consistent either. Sometimes she looks older, sometimes younger. If you consider this chapter alone, her height difference compared to adults most certainly wouldn't make me guess 9, no chance. I'd even say 14, though there are always individuals who begin to grow really early toward their final height, but maybe 9 would still be a stretch.
someone else elucidated on it already, but -
In that world/game/whatever, the development of mana within a person accelerates their growth - physically and mentally. It creates little magical tumors/"stones" within the body as well - we see that in her dark elf mentor, who was forced to develop multiple attributes, thus putting her life at risk because of the volatile mana stones in her body; and the "magic murder princess" we got a chapter of recently was the same, wherein she's doomed to die young because her father/family tortured and experimented on her with magical implants/surgery/"forced growth".
It honestly is described like a cancer - just like how tumors are mutated defective cells of the body that grow at exponential rates by consuming and stealing nutrients and energy from the body, mana sorta does that to the body, instead - I don't know how 'deep' the mechanics go in-story, but the way the stones and the accelerated growth are described, I think of it as the mana turning the whole body into a tumor, cell by cell - forcing accelerated growth and overproduction of energy, which manifests as the actual mana that is turned into magic by the person, but it forces the development/maturity of the person at the same time via that mass cell division/multiplication.
That's my interpretation of how it works mechanically, but we get the explanation in-story of how mana development "ages" a person beyond their actual calendar years.
So Aria looking the way she does is a result of the accelerated growth of her body from learning magic, on top of the survival conditions she did it under which likely exacerbated the process.
And, like how when you train and exercise and your muscles grow stronger and you gain physical aptitude, Aria constantly training her magic simultaneously with her body, compounds the process of gaining physical strength as well as magical ability.
She's still smaller than her opponents, though, so even though she could likely easily overpower anyone her size (like we see when knocks out the thug kid with her own fist before dragging him away in this chapter), she has to fight "smart" rather than "hard" against most enemies, which is why we see her setting traps, preparing poisons, doing tons of scouting, and relies on her "benign appearance" to lull her targets into a false sense of security by thinking she's harmless.
