Well, that's an interesting observation. According to Intel, the i3/5/7 processors support up to 64GB of RAM. However, Intel's given specifications are not only based on theoretical limitation, also on practicable limitations.
The theoretical limitation of DDR3 is 16GB per module (and thus 128GB for 8 modules). However, such modules are not on the market yet (they require 2x8x8GBit ICs and not very likely to be released very soon). The largest non-EEC DDR3 modules are 8GB and therefore the 64GB limit is a practical limit. Interestingly, as you can see in the MSI's certified memory list (http://us.msi.com/file/test_report/TR10_2765.pdf)
there are no 16GB modules at all so MSI doesn't have an idea either how to get 128GB RAM on that board!
I assume it is some marketing thing of MSI but practically there's zero difference between in the boards regarding the maximum memory.
If you, like me, however require a large amount of memory I would go for a different build. Something with a workstation/server- CPU, registered RAM etc. The Gigabyte GA-7PESH3 board, for instance, actually supports 128GB RAM and the Asus Z9PE-DP WS Supports up to 256GB (I actually made a quote for a customer build on this board last week).