Twi and AJ are better with medium to me. Rarity seems like she could dress to make ANY kind work, but I actually like her and RD with small. Flutters is big. I see so little art of her as medium it makes me sad. Pinkie...is completely bloody random.
Apparently 99.9% of humanoid pony artists disagree
There was an anthro dragon with nice boobs in one of the linked pics. Best of both worlds.
Hell, it's pinkie, she could probably change her breast size on a whim. One day she could be an A, the next a DD, the next a B and so on.