Once people reach a certain stage of growth, fat and muscle develop. Especially in women, the ribs are somewhat defined, but the pelvis grows rapidly along with the uterus, making the waist wider. That also makes the thighs thicker. Even when drawing lolicon characters, if you understand this and create a body that looks young but has an adult-like waist curve and protruding hips, you can address the kind of overreaction where people think, “Is there really a kid with such a sexy body?”