Megabytes of stats saved after every connection

Dear Postgresql experts,

For some time I had been trying to work out why every connection to my
database resulted in several megabytes of data being written to the
disk, however trivial the query.  I think I've found the culprit:
global/pgstat.stat.  This is with 7.4.7.

This is for a web application which uses a new connection for each CGI
request.  The server doesn't have a particularly high disk bandwidth and
this mysterious activity had been the bottleneck for some time.  The
system is a little unusual as one of the databases has tens of thousands
of tables (though I saw these writes whichever database I connected to).

Looking at the output of vmstat I could see about 2.7Mbytes being
written up to about 5 seconds after the query was processed.  I was
scratching my head about this for a long time, but today I noticed that
this size was just a little larger than my global/pgstat.stat file.  So
I turned off stat_start_collector and stats_row_level and the writes
vanished.  Turing them back on, the pgstats.stats file is much smaller
(10k) and the writes are invisible against the background noise.

So can I expect this file to grow again?  I think I need the stats,
though I'm not entirely sure about that.

Was the entire file re-written, even when the only query I've run is
"select 1"?  Is this necessary?

Any comments or suggestions gratefully received.

--Phil.