I don't think Rinpoo has proportions like that. Maybe after she becomes an adult she will, but the game makes a point of her being somewhat envious of Nina, who was a few years older. Her petite status also comes up at several other points in the game when the topic of a woman's body is relevant.

Sprites and concept art do not show that big a difference - they're both quite slim and buxom. Fanart likes to give her a more athletic build. Don't remember it being brought up in dialogue, but I've only played the original translation and even that was an awful long time ago, so I'm probably wrong there...