I would definitely go back to small rats. I don't feed mediums to any of mine except the 1800g+ girls, and that's usually only every third meal or so. It sounds like you're just hitting the feeding strike that females tend to when they reach ~1000g, and it doesn't help that it's winter, when older BPs tend to go off feed anyway. Feeding the larger meals probably "helped" her not be as interested in food - especially since you noted the large defecations, meaning she's passing a lot of nutrients instead of digetsing them. Long story short, I'd reduce offered size and not worry any time soon.