i bought the proper size and it is a great fit. my mare, although a tb, has quite a broad chest and hers is a good fit. it isn't the lightweight, it is the heavyweight stormbuster one. nice rugs, good delivery too.
My mare is 16hh and would normally be 6'3 but I always go for 6'6 anyway as I find 6'3's aint long enough. I got the premier equine fly rug and it fits her great.