I had an idea which I acted on · 6:00pm Feb 18th, 2013
I realized that instruments have to be a certain size to achieve the proper sound. Cellos, like Octavia's, are this same way.
If a cello goes up to an adult's waist (let's say 3 feet) and Octavia, when on her hind hooves, is a few inches smaller than that, it's safe to assume that ponies are really little- about 2 feet tall on all fours.
Aaah, science.