I'd check and see if they actually
are too skinny. Feel around their chests for the keel bone, it should feel like a half circle from the bone running down their front. If the half circle feels more like a divet with pudge coming out of either side, your duck might be a little overweight, If the bird has a prominent keel, they are too thin. Some birds naturally have a prominent keel, but in general- it is one of the better indicators of how thin/fat a bird is. The more prominent the keel, the less muscle tissue around it, hence the bird is thinner.
If they free range, they may just be getting a lot of their feed from other sources.
@Ravynscroft would this be correct?