That's astonishing to me! My sister has golf and her husband a Jetta. I love the golf but Jetta is, as designed, a bigger car. There are definitely loads that will fit golf and not Jetta due to different trunk entry ways, but I'm still surprised to hear that in terms of sheer volume, golf wins with seats up :O

Perhaps due to the storage space not stopping at the trunk lid level, but continuing vertically upwards?

Oh absolutely; it just always felt like what Passat gained horizontally offset what it lost vertically. Again, not that I'd go for a sedan over hatchback personally... and if it can't even make up the lost cargo space, the point of long sedans is even more lost on me.