If you bake, you know that the standard recipe calls for large eggs. If you only have medium eggs, or extra large, or jumbo, you're adding either too little liquid and protein to your batter or too much. Doing so will make a cake denser and gummier. When cookbook authors develop recipes, we try to use standardized ingredients that are available everywhere so that home cooks can have the same results we do. But a lot of people don't even look at what it says on their carton of eggs.