Size gender ducks

[IrisJade]

I have 3 Indian runners 3 Weeks Coming up to 4 this Sunday Ducklings and 2 look like they have grown more or just look bigger does this mean they are possibly male or does size be random?

 

[Yardmom]

I have not noticed size differences in my runners until they are almost fully grown. My guess is just a difference in genetics, but I'm just guessing.