SysInternals CacheSet has a limitation: it is unable to set a cache size larger than 4GB. This is due to the fact that it is a 32-bit application, and the respective API (NtSetSystemInformation) accepts new settings as a 32-bit byte count.
The solution: use the 64-bit API, which uses 64-bit integers. I’ve written a very simple 64-bit CacheSet-alike – just enter the desired cache size (in bytes). You can use the original CacheSet to check the new settings (just don’t hit “Apply”, or your settings might get clobbered).