psql memory usage when creating delimited files

I had in interesting issue the other day while trying to generate
delimited files from a query in psql, using:

\f'|'
\t
\a
\o out.file
select * from really_big_table sort by createddate;

This quantity of data involved here is fairly large (maybe 2-4GB).
Watching the memory usage, the postmaster consumed a fair chunk of RAM
(obviously) while running the query, but I was surprised to see psql
start taking increasingly large quantities or RAM, to the point that in
the end the machines memory was exhausted, postmaster died (and
restarted OK) causing psql to quit.

I was surprised in that I assumed that psql would just be taking rows
from postmaster and writing them to disk, hence requiring very little
RAM, but it appeared that it tried to load the data into memory.

Is there some option I'm missing in my export script that would prevent
this happening?  I managed to work around the issue by issuing a number
of smaller queries, but that's not something I want to do on a regular
basis.

Thanks,

David.