Hopefully there is nothing wrong with her and it's just genetics. Remember that even though you got them at the same time, since they came from a large hatchery, they could have totally different mothers and fathers. It's possible the smallest one is a female and had a dad on the small side and had a mom on the small side and that's all it is. So far they look like healthy ducklings. As long as she continues to grow and feather out normally I wouldn't worry too much. I agree with making a deeper water dish. I made ones similar to the photo above and used a gallon milk jug with a hole cut about 2/3 of the way up that they could get their heads in and it was big enough to last two ducklings until they moved out of the brooder.
