>>515
It's a tertiary sex characteristic: deposits of fat over the pubic bone that develop during puberty, similar to buildups of fat in the breasts, hips, and buttocks. There is also an element of growth of the outer labia. They don't really appear this prominent, so it's a form of hyper-sexualization - in context much like oppai loli - of putting an exaggerated post-pubertal characteristic on a character that is supposed to be prepubescent. In reality, girls this young have a very flat, unremarkable crotch.
There is also an element at work that comes from a much deeper part of the human brain. When some female mammals are preparing for ovulation (in heat) at a time usually once or twice per year, their vulva will swell much larger than usual, and in some cases becomes redder. This happens prominently in dogs and also some primates. So in a very primal way, an unnaturally puffy vulva on a female informs the male that she is ready to be mated, and it is therefore appealing. It produces greater feelings of arousal, and also an element of implied consent.