It isn't so much the physical limits but more the cost of manufacturing a physically more complex board. This is why most low end GPU's typically have their mem

It isn't so much the physical limits but more the cost of manufacturing a physically more complex board. This is why most low end GPU's typically have their memory bus width cut down to save on PCB and RAM complexity costs. An extreme example, the phone I am using has RAM running at 1.8Ghz but on a 16bit bus. Anything to keep costs down.