Certainly, if you don't like dressing up, don't! But if you do, and the extra luggage is the only problem, that's easy. One long skirt, and as many simple but dressy tops as needed, don't take up much room. Granted a man's tux or even suit would be more of a problem, but the solution is to split it --- pack for her, rent for him.
Almost everyone overpacks. If you can learn how not to do that, you'll have plenty of room for formal wear.