I mean... She isn't a fully developed adult... She's 15 or 16 if I recall correctly. But I get your issue. I dislike the trend too.
But, in this case, I like to think of her as short and without curves. I graduated from high school with several people like that. The key here is that she isn't treated like she's 11 and doesn't act like she is either.
Totally agree, but I feel like it's meant to give off that gap moe vibe where the MC looks like a sweet little doll, but is actually super dominant and charismatic. Meanwhile, the prince looks all mature and wild, but is secretly just a sweet, shy softie. Still, keep in mind they're only 15 and still growing. Human growth is different for everyone—some of us (me) don't really develop until adulthood, so it's no big deal, honestly

I really dislike how childlike the FLs body is. She looks like she is 11 years old as if she isnt fully developed into an adult. I dislike this trend but the series overall is interesting.