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You're right. I mistook the string implementation for the list one 
which does keep a preallocated section in case of growth. Strings of 
course don't grow so there's no need for that.
I tracked the memory allocation all the way down to 
obmalloc.c:PyObject_Realloc . The call goes to realloc(p, nbytes) which 
is a C lib call. It appears that the memory space is not reallocated.
That was enough to be able to find the python-dev thread "Darwin's 
realloc(...) implementation never shrinks allocations" from Jan. 2005, 
Bob Ippolito's post "realloc.. doesn’t?" 
(http://bob.pythonmac.org/archives/2005/01/01/realloc-doesnt/ ) and Issue1092502 .
Mind you, I also get the problem on FreeBSD 2.6 so it isn't Darwin 
specific.