Basically, each DRAM chip has a certain interface width. That can be either 16-bit or 32-bit, but nominally it's 32-bit. These chips are hooked up directly to the memory I/O controller on the GPU or CPU (console situation).
The 16-bit mode just allows one to use double the memory chips in the event that you want more memory, but the DRAM density isn't high enough. It was not uncommon for GPUs to have 16x 1Gbit GDDR5 in order to offer 2GB, but that's before the 2Gbit chips started showing up. Even then, 2Gbit is only starting to appear en masse. Anyways, that's just an example. Same thing happened for earlier generations.
So with that in mind, you have possibilities for memory configuration. For a console chip, the company is going to be mindful of the die size as well as a roadmap for future die reductions. The memory controller I/O width on the GPU or CPU is going to place a minimum die size restriction because that's one of those things that just doesn't shrink with process node.
You might think you can get away with a migration to a smaller bus whilst using higher speed memory in the future, but that's just added risk and QA for developers. Far simpler to keep everything as identical as possible when doing future redesigns.
Anyways, so you have a fixed memory bus width taking up a huge chunk of the chip perimeter, and also keep in mind they are likely going to have another huge chunk for the eDRAM interface.
Clearly, you don't want to have a huge perimeter necessitating a large chip throughout the lifetime of the console. That has a number of implications for cost/yield/power consumption (because you'd not be able to use a newer process node).
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Where the WiiU is concerned, 128-bit is a good starting point, but that means either 4 or 8 DRAM chips. Space is clearly very limited on the motherboard.
People hoping for GDDR5 will be limited to using 2Gbit density since nothing higher is on the roadmap.
DDR3 is far cheaper and much lower power consuming FWIW, plus there are 4Gbit densities available at modest clocks. Four chips would be very good for conserving motherboard space since you can put two on each side.
Electrical signalling is a bit of a concern for higher clocks when you're packing the wires very tightly, so... that's just another design concern.
Anyways, to achieve 1.5GB of RAM you have two choices, the first of which is the most obvious: Add two more 2Gbit DRAMs to the above for 6 chips total. The 16/32-bit modes is where the 96-bit/192-bit theories come from. If you use 4Gbit DDR3, then you only need 3 chips, so that's... 96-bit.
The second choice isn't so obvious because you almost never hear about such a configuration, which is to use a mix of DRAM densities. Consider that for 1.5GB of RAM, you really need 12Gbit worth of DRAMs. This can be done with 4 chips (128-bit) - 2x4Gbit + 2x 2Gbit. To my knowledge only one PC SKU has ever done such a mix of chips (
GTX 550, 192-bit, 1GB SKU).
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PC memory modules are a different situation with memory addressing, allowing you to hook up to 16 DRAMs on a DIMM to a 64-bit channel to the CPU. I digress.
