Most ducklings in stores are the White Pekin breed and are at pretty near adult weight. Some other breeds are smaller with the two color phases of runner the most common. Runners carry their bodies in a much more upright position than Pekin ducks, but I doubt that anybody could see the difference in a frozen carcass. The frozen mature ducks may not have any giblets and almost certainly don't have packets of horrible orange sauce adding weight but not value.
Cathy2 is certainly correct that these mature ducks are hens past their effective breeding lives. Hope that they aren't old drakes, which will be even tougher with stronger flavor. The light weight is a fairly good indication that they are hens because drakes tend to run slightly larger than hens. Muscovy drakes, though, run about twice the weight of hens. Muscovy hens run only a little larger than White Pekin hens.
I have seen mature frozen ducks at Tony's, too.
Moist heat cooking or grinding the meat before cooking are necessary. I have long suspected that old hens are used in a lot of duck ravioli.
I raised ducks one year as a teenager. Never again.