As you don't want to starve them, which is effectively what putting them on a bare pasture alone does, I would add some straw for them to eat. (Plus minerals, as mentioned.) This keeps their rumen bacteria going and will keep them filled while reducing their nutrition level.
If even this doesn't reduce their size as much as you would want, I wouldn't worry too much. They'll probably be OK and reduce in size after their first pregnancy. I had a pair of grossly obese Ryeland crosses, much like Marches Farmer's Southdown. I worried that they wouldn't get pregnant. But they did. I then worried that they'd get pregnancy toxaemia. But they didn't. They also had a set of twins apiece with no problem at all.