How to log to postgresql.log

Hi.
I'd like to log everything to PGDATA/pg_log/postgresql.log with
rotation when it reaches the size of 1kB (yes, that is not much, but
I'll increase when I see it work)
I have
these lines uncommented in the postgresql.conf file.

logging_collector = on
log_directory = 'pg_log'
log_filename = 'postgresql.log'
log_truncate_on_rotation = off
log_rotation_size = 1kB

But I get files

postgresql.log.1233263415
postgresql.log.1233263531

so it starts new logs each time I start the server.
Why? I don't want the extra part in the file name. How do I get rid of them?