I suspect something weird is going on. Try booting from the memtest utility. It's available from the RHEL repositories and pay particular attention to the "memory bandwidth" report.
If I had to guess, I'd say you've got e.g. 5300-speed RAM in both machines, but when the machine is filled with RAM the memory controller slows down the bus clock (presumably for power/heat reasons), actually slowing down the RAM. In these circumstances you'll get faster results with less RAM.
Compare the RAM speeds, it's almost certainly your CPU->RAM bandwidth.
Oh, I wouldn't pay much heed to the RHEL vs. SuSE debate on HPC use, I use RHEL-based OSes for HPC all the time.
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