Yeah, I think her figure is most probably quite model-like, especially thanks to that one episode... So I think I get why do you picture her as you do.
But in my head Rarity wouldn't go with just that with her design, as she'd want her friends' personalities to show in them as well. And that's where I got the flowy dress image. Kinda like Fluttershy's dress would be one with the ground...
But I got to agree, it's quite fascinating to follow how differently people picture those dresses. And some clearly have more effort than others, and that's why I like yours so much. x3