I actually have a good reason Koi looks so smol compared to the other shepherds else and I am surprised I didn’t think about it before holy shit-
So among a lot of thing I have made it pretty evident that Koi has a lot of traits that reflect the bond she has with the fell dragon. This being the fact her eyes seem more manakete/dragon like and the mark of grima on her left eye. . Thus the reason she look about 3-5 years younger than her actual age is due to the link she shares with Grima being stronger than she thought.
Furthermore, taking this into account with what Grima said about her not being strong before I like to believe that despite her actions in killing Grima herself that at one point she will be able to seal part of her power to create a dragon stone and in turn take up a dragon form herself.
Thus long story short. Koi is more manakete/dragon than human.