Yyep... Anna (Yamada) was a tall girl. Go back and check out
Ch6.1, and you'll see the earliest sign of this, at least in my case of reading this for the first time now. That splash art was a wild visual, and helped to illustrate that, yes, she's been a giant since puberty hit in or just before 5th grade. You can also see she's already got some slight bumps on her chest coming through her shirt, to indicate she's already developing in that way at that time...
So yeah, to me, it was a foregone conclusion she had tall person genes, and that also meant tall person metabolism, which explains the snacking (a strong appetite) and the fact she doesn't seem to gain weight despite her gluttonous ways (metabolism burns it all). This is because I also benefit from tall person genes, and so does at least one of my sisters. Though she didn't get nearly as tall as I did, she is about as tall as Anna is, and my sister is an adult. There's still a possibility that Anna will keep on growing through her teenage years, as I kept growing up through into my early 20s into the 6'7" (2 meter) giant that I am today. With nary any body fat, and an inability to really gain much of any because of my sustained metabolism.
And yeah... Kyo with the 'ow the edge'. ... also, why doesn't their mother call her Kana? That's her name. Why is their own mother calling her 'big sister' instead of the name she and their father gave her?!