postgres writer process growing up too much

Hi to everyone

I've been monitoring my postgres processes and I noticed that the resident
memory
size of the writer process is growing up too much.
Those are reports from top in a 24 hour interval:

YESTERDAY:

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
11706 postgres 17 0 157m 148m 146m S 0.0 3.9 3:23.41 postgres:
postgres BJ3 127.0.0.1(64648) idle
12174 postgres 16 0 157m 148m 146m S 0.0 3.9 3:17.58 postgres:
postgres BJ3 127.0.0.1(65367) idle
11495 postgres 16 0 157m 148m 146m S 5.7 3.9 7:51.98 postgres:
postgres BJ3 127.0.0.1(64603) idle
11908 postgres 16 0 156m 147m 146m S 5.7 3.9 7:35.84 postgres:
postgres BJ3 127.0.0.1(64947) idle
11419 postgres 15 0 155m 112m 112m S 0.0 3.0 0:00.49 postgres:
writer process

TODAY:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
11706 postgres 17 0 157m 148m 146m S 0.0 3.9 7:37.16 postgres:
postgres BJ3 127.0.0.1(64648) idle
12174 postgres 17 0 157m 148m 146m S 0.0 3.9 7:40.10 postgres:
postgres BJ3 127.0.0.1(65367) idle
11495 postgres 16 0 157m 148m 146m S 6.3 3.9 17:46.99 postgres:
postgres BJ3 127.0.0.1(64603) idle
11908 postgres 16 0 156m 147m 146m S 5.7 3.9 17:29.31 postgres:
postgres BJ3 127.0.0.1(64947) idle
11419 postgres 15 0 155m 140m 139m S 0.0 3.7 0:01.09 postgres:
writer process

Any ideas why is this happening?
My postgres version is 8.1.3

Thanks a lot
--
Heiner Vega Thames