i have a similar problem with some wild ducks that I feed each day.
There is a little duckling which was either abandoned by its mother or the mother was hit by a car on the road. In the last week I have realised that it has not been growing much. I know its not growing anywhere near as fast as the others, as there were 6 ducklings (and their mum) that Ive fed regularly every day for a while, and they went from yellow and brown fluffy things, to looking pretty much like adult ducks in 4 weeks. This one must be close to that age now and it still has the fluffy baby duckling feathers and looks like its only a week or so old.
Like others have said it seems healthy and has an appetite. I thought maybe it was not getting enough nourishment and this was delaying its growth? It comes and joins in with the larger ducks at feeding time, but they will chase it away. i try and 'sneak' it some food, throw it to it out of sight of the larger ducks but it can be difficult to do that. Being a wild duckling, catching it, and feeding it more is out of the question as its very easily scared and wont come very close to me.
I have managed to 'sneak' it some food in the last couple if days and its getting more used to me, and seems to have figured out that if I quietly crouch down with my hand out, it should come towards me. And just now I managed to get it to come close enough to me, around the side of my van that I could throw it some cat food without the other ducks seeing and taking it instead.
Does anyone know if this could be the reason its so small, not enough food? Maybe its natures way of dealing with a lack of nutrition, keeping the size small so its needs are less?
It migt also explain the extremely rapid growth of my 6 ducklings (who come to the door and 'talk' to me if I havent feed them when they feel I should have). They are all now doing a lot of wing flapping, so Im hoing to see them fly soon!