how to make egg casserole fluffy

Besides the good advice HalfPint gave, you can even go a step further and beat a couple egg whites and fold them in.

Beat a lot of air into the eggs, until the eggs are foamy. When you bake the casserole., the heat will make those air bubbles expand and your casserole will rise. You might also want to avoid overloading the egg with too much add-ins like meat, vegetables, cheese, etc.