In the normal installation logfiles for squid are rotated an compressed. This has to be changed. The System has to store more files, store them 30 days uncompressed. I used Debian Wheezy and Squid.
File: /etc/logrotate.d/squid
##### Values changes to daily
/var/log/squid/access.log {
daily
rotate 30
copytruncate
# compress
notifempty
missingok
}
/var/log/squid/cache.log {
daily
rotate 30
copytruncate
# compress
notifempty
missingok
}
/var/log/squid/store.log {
daily
rotate 30
copytruncate
# compress
notifempty
missingok
# This script asks squid to rotate its logs on its own.
# Restarting squid is a long process and it is not worth
# doing it just to rotate logs
postrotate
/usr/sbin/squid -k rotate
endscript
}
Source: http://www.faqs.org/docs/securing/chap28sec234.html