If you ever get a chance to open a DSLR and look inside you will be amazed to see how tight everything is packed in their already. Some of the larger bodies have two functional card slots which provides some benefit (can do jpg on one card, raw on the other for instance), but space is at a huge premium. If people don't have enough room to carry a spare card, it is unlikely they will bother carrying a DSLR. If size is really important, nothing beats a PnS.