I believe the egg stays consistent size for the female. Maybe someone else will chime in, I’ve never crossed duck breeds.OK will definitely be using a call Drake on a runner hen! Will that make the ducklings be bigger though? Will they be small and make it harder for them to get out of the egg.
I do have one large chicken hen that has produced chicks with bantam roosters. Her eggs stayed the same, chicks are much smaller than her. She has hatched a few, and from what I’ve seen, they hatched fine. But I could also see it being an issue.