I've seen some recipes include Cream of Tartar with the Egg whites, or using a copper bowl. This should help with height a little bit.

That's the nature of souffles, in my experience. It's part of their allure.

The reason is because the bubbles in the egg whites heat shrink when they are cooled. It's a normal process, heat expands them, cold shrinks. However, they will live to rise again, if simply reheated. Hope this helps!!