I prefer wide back but do find that many of them shrink quite a bit (especially muslins) and frankly, think there is no way that it can be wound on a bolt without a fair amount of it being out of square--so I always over estimate when purchasing. I saw the left over and use it for string quilt foundation, etc.
But that does seem more than normal shrinkage and doesn't sound like you caused it by the way you washed it either.