Interesting that the Sienna has more volume space than a Grand Caravan. I did not know that. Maybe it has something to do with the raised floors and excess plastic taking up space in the DGC. Years ago I skipped on the Sienna because it could not hold 4x8 sheet goods and close the trunk.
Technically behind first row, cubic feet:
Dodge Grand Caravan 140
Toyota Sienna 150
MB Metris SWB Cargo 183
MB Metris LWB Cargo 199
MB V class ( I just learned it exists in three lengths now ) 165 based on 4680 liters for the " regular " length as we'd know it in NA. ( DE : Kompakt, Lang, Extra Lang )
The biggest advantage / difference of the Metris over other minivans should be interior height ... and compared to the Mopar vans: a flat floor. I don't know about the other minivans, been too long ago I have been in those. But my DGC was kind of a pain with its undulating floor.