Okay found it!!! It wasn't confirmed but only suggested by her dad.... the growing due to imprinting was completely wrong which was my fault. No idea where that even came from lolol
But I did find the part where her dad says to her "my daughter has become even more affectionate. It seems I might have become what people call 'marked'." (Aka he mightve been imprinted by Ayrin)
Based on how she's been acting around her dad, I'd like to say she has unconsciously imprinted on him already and that moment hinted at that which is in chapter 68
i'm pretty sure they said when a dragon imprints is when it starts to grow and develop its powers, and i'm pretty sure she imprinted back when he picked her up when she ran away the first time. i remember they said something about when the dragon's affection is reciprocated is when they imprint, and when her dad went to go bring her home after she was forcibly transformed and then went to that orphanage and was kidnapped and almost sold, they decided to be a family for real, acknowledging that whey were pretending before but it wasn't pretend anymore, and she started eating a bunch when they got back and her dragon form got bigger and she got those nubby wings lol.
I'm literally rolling my eyes at all the people in this manhwa who continue to wish for Ayrin to be "theirs" like she hasn't imprinted on anyone. Didn't she already imprint on her "dad"? I mean.... that's why she finally grew to look her age
Edit: Oops. I was wrong that she grew due to imprinting. She grew because she "wished" to look her age. However... I'm sticking with the imprinting guess