This is a rough list just by a quick passing glance. Lol! Biggest at the top, smallest at bottom. This is how I determined what I did.
Here. I did not count the original AT&F series 1 comics. (When she was rounder and non show styled) I used side, front views (seeing how much sticks out) and back views to make a judgement, though i didn't spend more then ten minutes on this and it's not really in depth.
MARES
Twilight
Starlight Glimmer (She's actually got a pretty big one in this series)
Octavia (It can be the largest, but it depends on the artist who draws it)
Vinyl Scratch
Applejack
Rainbow Dash
Lyra
Rarity (It is soft though!)
Pinkie Pie
Bon Bon
Cadance
STALLIONS
Big Mac (It is square though)
Shining Armor
Soarin'
Greg
Spike
I did pretty much just say I think there are at least two different artists who draw this series. One of the artists is more show accurate, and the other or others are not and a tad more messy in style.