Total usage is currently 260 MB.
Forum software is UBB 5.47c from InfoPop, though I've rewritten about 3/4 of it, and it does not use MySQL. Messages are stored as simple text files. I have nothing against MySQL, but one disadvantage is that many webhosting companies charge additional for it. For example, our current webhost charges $15/month for MySQL.
I'll soon be releasing a new revision that reduces disk usage, possibly by as much as 40%, but we'll have to wait and see the final result.
I plan to look into compression options next. There's an Apache module called mod_gunzip that I plan to investigate (but I think I'd have to persuade my webhost to install it), and if you know of any other viable options I'd appreciate knowing about them.
Another option is to simply not store static HTML files but instead build them dynamically upon request. This would place a greater demand upon the webhost, but hey, maybe it would encourage them to make disk space cheaper.
--Percy