This is actually a very common trope in manga.
Just like you have the adult lady who looks just like an underaged loli, you also have the grade schooler who (due to a precocious growth spurt) has the looks and the body of a foxy sexpot bombshell, with bigs boobs and a bubble booty. I've read dozens of stories with this premise, both shoujo and shounen, oneshots and long series.
I remember one where the MC (the adult-looking kid) falls in love at first sight with a cool, hunky sarariman in his mid-twenties. She thinks it's hopeless... but then this other girl (her best friend and classmate) tells her that, with her looks, she has a chance: all she has to do is pretend to be a grown-up! With the right clothes, accessories and makeup, she can pass for a super sexy twenty-year-old woman! And so, long story short, MC gives it a try and it works like a charm: the guy falls head over heels for her. And even though the truth is eventually discovered, he forgives her and they start officially dating with the blessing of her parents. Oh, btw, did I mention that this manga was published in a shoujo magazine for little girls? That's Japan for you, lol.
I think this is the first time I do find this trope in a yuri story.