A girl duck says "QUACK" and a boy duck says "wha".
Like this:
When they get older boy ducks will have curly tail feathers.
Some ducks who are lower in the ranks get less time at the feeder, and that can account for size difference. The Cayuga may be a pip-squeak, too.
Good luck.
I think hatchery stock can be very variable where "type" is concerned. You may be witnessing that first hand.
Or, you may have a drake?
I am not sure. I just hate the zero. I know how hard it is to wait for responses.
Remember, too, every duck develops differently. If the Cayuga is lower...