My tux and my wife's formal dress would fill up a carry-on.
You're right about that. Hubby usually wears a dark suit on formal night, where he packs the trousers and wears the jacket on the plane. The one time my husband brought his tux was our cruise on Queen Mary 2, to which we were able to drive, so the extra luggage didn't matter. I wear fancy pants suits on formal nights and those pack easily.