If the pasture is rich, you may need to restrict their access to it, to avoid them becoming obese or insuline resistant & prone to founder. However, horses are designed to have food going thru their system pretty much constantly, prone to colic & other digestive problems if not, so be sure to provide hay - or at least straw for fatties - if you're going to corral them for half the day.
I would be calling a vet to do blood tests if you're concerned, as it could be a nutrient imbalance or something else causing the muscle wastage. However, if it's only at their wither, I'd be inclined to suspect it was saddle related. Do they move any differently under saddle?
More often than not, saddles are not built for a horse's comfort and can easily cause muscle wastage in a very short time - even hours or days. Even a good, well fitted saddle should be checked every few months, as horses and saddles both change shape with time. Perhaps with the weight gain and regular work, the saddles have started pinching them.