Your correct Peter, though I personally think allowing stacking of items is a good idea its really a separate idea/request that I'm sure has threads of its own.
Hypothetically the 'big' grids would allow normal items to be stacked up to some limit based on their size in the normal grid, so you could get say 3 boards stacked in a big grid cell but 12 straw. I don't really see any reason you shouldn't be able to load up something like fifty boards directly into a wagon, if anything the wagon should hold more when no containers are being used.
Would this allow containers in containers (in containers)?
No more then is fundamentally possible now with Boat->Urn->Seedbag, it would just be more consistent as each level would work on the same grid system.