Question with hashed IN

I've noticed that when the stats are wrong (like
in cases where you've loaded data but reltuples
hasn't been updated yet) that a hashed NOT IN
seems to take a significant time penalty.  Is
this to be expected?

On a pktest table with 1 million integers and a dual table with a single
integer and sort_mem set high enough to give a hashed subplan for the
various reltuples values, I saw the following behavior for

explain analyze select * from dual where a not in (select a from pktest);

with reltuples=1000 for pktest, query takes about 96 seconds
reltuples=10000, query takes about 15 seconds
reltuples=100000, query takes about 8 seconds

And the memory usage seemed to be the same even if I set sort_mem back
to 1024.