Hosting even a small forum (either php or asp powered) requires more server resources than a normal HTML website - everything is stored in a database (usually) and these database queries are what causes the extra server load... Every post & thread is just an entry in a database table and when you view every thread or post on a forum the software has to construct the page from the database...
Ever wondered how the
REALLY BIG forums cope with their hosting needs?
http://www.big-boards.com/servers.php
The "big boys" are running multiple servers for the web side of things and some are even running multiple servers for the database... There is some SERIOUS MONEY being spent in keeping these communities running!