Small duckling, worried

[NatC]

I have 5 runner ducklings, 1 is about half the size of the biggest duckling. It appears in good health other than being small. Anything I need to watch out for?
Could it be that 4 are boys and this is a girl?
They are 18 days old.

 

[Welshies]

Size is not relative to gender, although at 4-6 weeks you can begin to tell gender.
I had one duckling like that, too. She was a beautiful little silver hen whom i absolutely adored as she cuddled with me.
As long as they're eating alright, moving fine, and active, with no signs of illness there is no need to worry