Why would this use 600Meg of VM?

Can one of you knowledgeable people tell me why current CVS as of 
a week ago would have the backend running this query grow to 
600 meg+?


INSERT into traffic_summary
SELECT asn,protocol,
cast(sum(pkts_src) as float) as pkts_src,
cast(sum(pkts_dst) as float) as pkts_dst,
cast(sum(bytes_src) as float) as bytes_src,
cast(sum(bytes_dst) as float) as bytes_dst,
cast(sum(secs_src) as float)  as secs_src,
cast(sum(secs_dst) as float) as secs_dst,
min(early) as early,
max(late) as late 
FROM traffic 
WHERE early between '2001-06-01 00:00:00'::timestamp and                   '2001-06-18 23:59:59'::timestamp 
GROUP BY asn,protocol,date_part('epoch',early)/60/60;
-- 
Larry Rosenman                     http://www.lerctr.org/~ler
Phone: +1 972-414-9812                 E-Mail: ler@lerctr.org
US Mail: 1905 Steamboat Springs Drive, Garland, TX 75044-6749