Apologies for the long delay.
What it means is that /var/www/httpd/access_log
contains all server accesses to all sites.
All of the sites have their own httpd logs in /var/log/ispconfig/httpd/<sitename>
In a normal situation, if the individual sites had their own logs (error and access), then they wouldn't be logged to the system httpd log. In this case, they're duplicated.
Right now, I have four system access_log files, and they're 1193916 KB. (approximately) This is the same size as if I went through and concatenated all of the individual site logs, as if I was going to make a 'system' webalizer site.