There is nothing intrinsic in HyperSizer as it exists today that takes particular advantage of multiple core processors. The speedup going from one core to two is not surprising because with one core, all of the processes on your computer had to share that single core. Going to a quad core might see an additional benefit as well. We use quad cores in our office and they do a good job. You can also submit multiple HyperSizer jobs at once and each one will run at full speed where as with fewer cores, they would all be slowed down because they would have to share CPU. We are currently looking at some of the newer technology (E.g. .NET) to try to take advantage of multiple cores and parallel processing (in additiona to 64 bit) for some of the more intense processes that HyperSizer does.
I hope this helps.