The problem is that the maximum size of a ramdisk, more specifically of size of memory that can be accessed via the ramdisk driver is configured at compiletime, can be overwritten at boottime, but remains fixed once the kernel is loaded into memory. The default value is probably measured in Megabytes. If I recall correctly the memory for a ramdisk is reserved right when the driver is loaded, all ramdisks are the same size and there is are some 16 ramdisks by default. So not even you want a ramdisk size of 16G :-)
As stated in the other answer, tmpfs is what you want to use. Further, you won't win a lot by having your entire OS in a ramdisk/tmpfs. Just copy your builddir to a tmpfs and do your compiling then. You may have to ensure that all temporary results are written to a location thats in the tmpfs as well.