Purebred Mallard drake weighs 40 ounces, Purebred Rouen drake weighs 10 pounds.
A Rouen has a very heavy keel that nearly drags on the ground, making a Rouen box shaped. A Mallard doesn't have a keel.
The "Rouens" from hatcheries are some sort of gray colored mongrel, so no telling what you might get.
Just because two breeds are the same color does not make them similar in any way. Runners also come in gray color and they are nothing like Mallards or Rouens and Mallards and Rouens are nothing like each other. Lots of mutts come in gray color and that doesn't make them anything but a gray colored mutt.