i wrote a small thesis explaining why each of them has the size they do. fluttershy's are largest because she is the most fertile, sunset's are second because she is a grown woman. sci-twi and rarity are the next two because they likely eat very healthy, but do not partake in much physical activity. AJ, Dash, and Pinkie are the bottom 3 because they are all extremely active, Dash being the largest of the 3 because she likely eats the healthiest, and Pinkie being the smallest of the 3 for the opposite reason