This is all about installation vs a running installation within a virtual machine.
When you install Windows Server 2012 (R2) you will need to configure 800MB+ of startup RAM.
If you don’t, the installation will fail because there isn’t enough memory available.

But the memory requirements for a Windows Server 2012 installation is 512 MB, right?
So why would it fail?

The installation environment does its work fast and part of the reason is that it’s memory consuming.
After the OS is installed you can run the VM with less memory, depending on what you’re running on the OS.

Note: All this can be found in the ‘System Requirements and Installation Information for Windows Server 2012 R2‘ Technet page, under the chapter about RAM.