One of my Indian Runner ducklings is a runt. It is quite apparent now as the others are about twice her size! It looks weird. Does this mean anything? Like is it a male/female? Or just crappy genetics?
I had two mallard ducklings and one was half the size of the other. She was a crested female, so it was probably genetics in that case (cresteds are prone to genetic problems). She is doing fine so yours will probably be ok. It does not tell you anything about the gender; full grown mallards are all the same size no matter the gender. The size is probably involved with genetics. You have nothing to worry about though . He/she will still probably be healthy just a little small.