It is intentional that atlas now defaults to clang; see r104549.
Yes, it's known that clang can use much more memory than gcc in some circumstances.
MacPorts usually starts multiple compiler processes, and by default it limits this to one process per CPU core or 1 process per GB of memory, whichever is less. But these limits were decided upon before we started using clang. Perhaps we should decrease this to one process per 2 GB of memory when clang is in use.
Individual ports can override this e.g. using use_parallel_build no to turn off parallel building entirely, and to my surprise, the atlas portfile already does this. So either the atlas build system is taking matters into its own hands about how many jobs to start, in which case it should be disabused of that notion, or a single clang process is taking that much memory, in which case that's very unfortunate.
