Actually, it doesn't really work like that. Each page is equivalent to 150KB to 200KB. They're really not that big. Also, the layout of the site, including some of the pictures, are cached. What is cached? That means that your computer keeps a copy of the site's contents so you don't have to redownload it, thus saving page load time and bandwidth. Plus, since most forum software uses PHP, there is a compression feature that you can use to save even more bandwidth(pretty sure most people are using it). I would say that a forum with frequently used attachments will use a ton of bandwidth, though.
I'm planning to sign up for the X10 business server in the end of this month. All the other hosts don't look as favorable as x10hosting. Sure, some might have high tech security sites, backup power generators, and all sorts of gadgetry... but is it anything if you don't have a good community to ask for support? Nah.
The only thing I'm afraid of is whether x10hosting will run out of business because all of the costs and time(a very big deal) used on the free server.