Aright guys, here's my take on the human-pony boobs question.
Largest: Celestia. Already has the most "adult" body type even in pony form, also princess/queen archetype, so I'd imagine a very "mature", curvy sort of body, helped along by a royal lifestyle.
2) Luna, Nightmare Moon form. Same reasons, she's more warrior-ish with her armour which would make her less curvy and more athletic but, at the same time, "evil is sexy" so she'd be unrealistically endowed. However, she may have had emaciation as a sign of her imprisonment if she were human.
3) Luna, Princess form. Maybe? Smaller now, but still larger than normal ponies, so that could translate to a human mode. On the other hand, she might be made slim to fit the little sister type.
4) Mayor. An older lady with a sedentary lifestyle, also an Earth Pony. For some reason I think Earth Ponies will be more stocky, either in "hard" musculature style and the "soft" curvy or plump style.
5) Pinkie Pie. Again, Earth Pony, also stereotyping (she's already pink, do you really think they'll hold back now?) of the party girl/ditz.
6) Applejack. Earth Pony, probably well-endowed naturally, exercises hard but, and here's the thing, probably well-fed at the same time. You can be healthy and well-muscled and still have weight on you.
7) Fluttershy. Fits the nurturing, hippyish sort of character type, and she doesn't strike me as being too physically active.
8) Twilight Sparkle. Yes, she's nerdish, but the "thin nerd" stereotype comes from the idea that the person is too focused/obsessive to remember to eat, and Twilight has been shown being bothered by her hunger rather than ignoring it.
9) Rarity. If I can see any of the ponies deliberately trying to slim down, it's her.
10) Rainbow Dash. All implications have been that she's naturally talented for flying, and that would involve being lighter and more streamlined.
Now, I need to get back to work. Maybe then I'll feel like I'm not wasting my life!