Somewhat paradoxically, PyPy always uses more memory for programs with small working sets, but can use less memory for programs with large working sets. 1/6 is a lot more extreme than I would have expected, though.

I suspect their relative memory efficiency depends on the size of the program and the size of the data it's processing.

No, it has drastically less memory requirements. It's very similar to my perl compiler, and even in non-optimizing mode it uses much less memory. In optimizing mode it's almost the same as this one. Just ~20 years older.