.Net Why can't I get more than 11GB of allocated memory in a x64 process?

It's your machine.

I have an x64 with 8GB RAM and 12GB pagefile, and I ran your program and it topped out at 16.23GB.

EPILOG: Then my Win7 install gradually slid into a coma as critical processes were apparently memory starved.

EDIT: If you want to understand how Windows allocates (i.e. reserves and commits) memory, read Pushing the Limits of Windows: Physical Memory and Pushing the Limits of Windows: Virtual Memory.

Since .Net relies on Windows to manage the memory it uses to build the GC heap, the mechanics of how Windows does this are reflected in how memory is allocated in .Net on a low level.

It's up to 8 TB, not 8 TB. You can potentially have up to 8 TB, but you need the matching RAM/swapfile.