I agree that restoring the string interning behaviour would be a good thing.
However, I don't agree that supporting recursive objects and instantiation is useful. marshal is specialized for code objects, and you shouldn't find any recursive constants there.

As for the size of pyc files, who cares? Memory footprint may be useful to shrink (especially for cache efficiency reasons), but I don't see why we should try to reduce the size of on-disk bytecode. And if we do, it would probably be simpler to zlib-compress them.