folks, certain characteristics occurred in various areas due to availability of materials, type and condition of water, and even species gunned. For example, given that the upper bay decoys were made in the hundreds of thousands , and a lot were hand chopped before the advent of the duplicating lathe, and that they were mad to endure adverse conditions, especially during market gunning days, and in sinkbox rigs, and that the dekes often took shot charges, you will find that pretty much ALL upper bay dekes were solid.
That said, the folks in jersey who gunned tidal guts in small boats, or even pond boxes, needed lightweight dekes, which could be dragged or carried in the decoy rack of a bbsb, thus the necessity for hollowing out decoys.
I am sure most of you could cite other examples--for example, the bobtail diver so prominent in michigan.
Think of the dekes in your own region, and figure out historically, why the evolved as they did.
Tout a son gout.
as an add-on---i am pretty sure that materials used were based upon availability in that particular region. Areas without an abundance of wood for decoy making, and i am thinking of long island, as an example, seem to have relied on "odd" materials early on, such as cork. Places that had abundance of cedar used that, sugar pine hete and there, ship masts, whatever was easy to scrounge and either cheap of, better yet, free.