Just an idea, I suppose. Personally I like my human pony figures as such...should anyone care...
I mean I know you don't, but I'm going to type it anyways...
Rainbow dash is fairly small chested, but there's still something there. Shorter than the others with an athletic figure.
Applejack Would have some muscle. Not to go so far as a bodybuilder, but she'd be fairly toned up with the obvious farmer's tan. Chest size would be slightly above average to play with the cliched farmer's daughter look.
Pinkie pie would have a healthy figure, sporting a nice little bit of...extra...while still making it somewhat believable that she can bounce around and do the things that make Pinkie, Pinkie...
Fluttershy, for some reason, I see as taller than most. Fairly average hourglass figure, think of that classic sexy teacher idea, and add a little height to it. I don't really like the idea of giant chest Fluttershy...just don't sit right...Plus it would make the younger fluttershy the beanpole she appeared to be in the cutie mark chronicles.
Rarity has Fluttershy's Figure, just shortened down to the average height. Maybe a little bustier, but no real difference.
Twilight. A little extra from staying inside reading all the time, but not as much as pinkie. Shorter than everyone but Rainbow Dash as far as height with the appropriate bust size for someone with a little weight (plus a little extra for the sake of it.)