I think its her malnutrition and the special species
It's that manhwa height logic at work where boys are always a foot taller than girls before puberty
I'm not sure how well that arguement is gonna hold up because the story rarely references what meals she had and they never really actually explain the special species aside from a few lines of dialogue but also the visual imagery of her being below dick height and practically a third of his height/size is really off putting considering that she is going to get hitched to the guy. It's kind of like making the 1000 year old loli arguement like yeah in context this "isn't pedophilic" but the imagery kind of speaks for itself when my knee jerk reaction is "get the fuck away from that child" it doesn't help that she's really naive and ignorant adding more to her innocent childlike vibe while her love interest is a slimey little shit who thinks she deserves to eat and die cause she used to be poor and it shows.(but he's pretty so the story forgives him for being an asshole because that's shoujo )
Basically the story keeps not only implying that she's a child through her design but also through the way she acts and thinks( the way she thinks and acts is different from all the other characters because she's "pure") . The story is basically screaming, crying, whispering, muttering, speaking all to say this is an innocent little child.
I don't know if it's something wrong with the adaptation from novel to comic but the height difference between FL and ML currently is extreme because she looks like a preschool and he looks like he's in the middle of high school. Her head heightwise is way below his hip height. She is shorter than dick height WTF. This feels illegal.