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Ok.  I've tracked it down.  realloc(...) on Darwin doesn't actually resize 
memory unless it *has* to.  For shrinking an allocation, it does not have 
to, therefore realloc(...) with a smaller size is a no-op.

It seems that this may be a misunderstanding by Python.  The man page 
for realloc(...) does not say that it will EVER free memory, EXCEPT in the 
case where it has to allocate a larger region.

I'll attach an example that demonstrates this outside of Python.