I am seeing a problem with web.log size too.
These are the logs located in
My November web.log is 3.8GB
My December log is already 1.5GB
I have logsize set to 5 in ISP Manager but it doesn't seem to matter.
There are no errors in the cron log to indicate a problem with webalizer:
Dec 9 04:00:01 server1 crond: (root) CMD (/root/ispconfig/php/php /root/ispconfig/scripts/shell/webalizer.php &> /dev/null)