I have a big chested mare, and the Masta rug I bought her ALWAYS slipped and rubbed her shoulder..... I personally don't like them.
I love the Premier Equine rugs - they fit really well and do not move, however, they ARE expensive....
Weatherbeeta rugs are also very nice rugs, they are a nice fit and do not move, they are less expensive....
Horseware Amigo rugs are very nice rugs too.... and they are more affordable.
I do tend to go for the more expensive, named or branded rugs, as I find the fit/quality is better, and you do tend to get what you pay for.
Buy the best that you can afford.....:happy: