The following is not based on knowledge of the specific architecture of the Mac Pro, but general computer knowledge. Disclaimer over.
These CPUs support hex-channel memory. This means that the CPU will have six channels of communication to the memory. This can only be achieved if 6 or 12 slots are populated, so the CPU has either 1 or 2 DIMMs per communication channel. If 4 or 8 are populated it'll run in quad-channel mode.
Let's pretend we populate 8 slots with 64GB sticks, and 4 with 8GB sticks. If we put 1GB of data in each DIMM and try and access all that data, all at once, this will actually be faster than if we had only the 8 64GB DIMMs, since we can access the 1GB of data on 6 DIMMs simultaneously, versus putting the same amount of data on only 4 communication lines.
However, if we load up all the memory modules to the brim, the larger memory modules will obviously be used much more than the smaller modules when accessing all the data, since they just hold a larger percentage of it. This means the communication lines will be used unevenly and it won't scale the speed of communication as well. It in most any situation won't be slower than the case with only the 8, should in fact be faster, but not as fast as fully spreading the load across all 6 channels with evenly sized modules.
The very worst case would be having a single huge module and 5 tiny ones. Since almost all data would be stored just on the huge one, and not so much on the five smaller ones, communication from the CPU would almost effectively become single channel. - The point is more that if you want 256GB of memory, it is better performance wise to get it in 6 or 12 evenly sized modules, than it is to get a few huge ones and load the remaining with small ones, so they all can give and receive data simultaneously
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On the back of the memory cover the diagram of configurations is shown
support.apple.com
