Only using 33 bits for memory addressing is troublesome. 33 bits is 8GB ram which is small potatos for a desktop. Why couldn't they have left it at 38 or even 40 bits? Or is this limitation only part of the objective-c runtime?
It comes down to the OS. Basically, when a new process is created, the OS sets up its address space and decides where it will allow new memory to be mapped. For whatever reason (I'm not entirely clear just yet), iOS 7 in 64-bit mode goes for an 8GB address space.
As far as I know, there's nothing preventing that from being increased on future hardware or even on the 5S with future OS updates. I believe the CPU itself supports a 48-bit virtual address space.
