If she's an easy keeper, then I would not give her any oats or only 1 lb. Is there a reason you give sunflower seeds? They are very high in fat and Omega 6 fatty acids. They have almost no Omega 3's, which are more beneficial to the horse. I prefer flax meal for fat and omegas, just 1/2 cup a day for "easy keepers."
To your oats and flax, I would add a vitamin supplement designed for horses that get little to no fortified feed like GrandVite, NutriPlus++, Uckele's EquiBase Grass, etc. That will round out her nuturition.
If she loses weight on less oats, then increase her hay. Hay is what keeps horses warm during the winter months.