I have to say I HATE buying horse rugs! Being in NZ the measurements/sizing are different but I always struggle to get the right size the first time, always end up having to go back and swap it no matter how much measuring I do. Definitely the build of a horse makes a huge difference, I have a rug I used on a 15hh quarter horse, tried it on my 15hh paint and it's at least 2-3 sizes too big for him. He has a quarter horse butt but quite narrow at the front.