In real life there are people with small build with baby face. I have a friend who is 140 cm tall, weight under 40 kg, with baby face and people often mistakenly thinking her as junior high schooler even thought she's in her 30s.
And in this story case, if you're being malnutrition since young age (especially baby-toddler), it can cause stunted growth or they called it stunting. Stunting is when a child fails to grow to proper height for his/her age.
I know, I have a baby face, my weight is 38kg and I'm also very petite. I get mistaken as a 16 yr old even tho I'm in my 20s.
What I'm saying is, they should have drawn her in the earlier stages with gaunt cheeks and looking like she was actually abused instead of just looking dirty. There are webtoons that have drawn the MCs looking like skin and bones with chapped lips even and not the anime face so they actually look abused and not taken care of and then gradually they start to look better.
i know she's supposed to be malnourished but they could have drawn it in a better way rather than making her look like an actual child.. Like show gaunt cheeks, ribs peeking out or clothes being severely oversized on her. Actually make her look weak and then as time passes show her looking more healthy but her looks should match her age, no?