PHPBB is notorious for security problems. It's a GPL package that has oodles of features and has been creatively-cooperatively coded by a large online community, but it has been subject to waves of DOS and spam attacks over the years.
That's not unique to PHBB, but is generally more prevalent among open-source boards than commercial ones.
I started a forum on PHPBB a few years ago, but after a local server using the same system got trashed and had to shut down, I switched to Invision board (IPB). It's had its own share of problems, but has managed to react to security issues much faster. Ali might be able to get a translator to port over to a different system if the software's the problem.
But I appreciate the bandwidth problem. While my forum is much smaller - about 1,500 members - it sometimes caused problems for the local host. Later least year, I ended up switching to a larger, outside server who could handle the traffic better.
Yes, the files must be huge! My own board exports the full MySQL database at about 120-130 MB and growing - the Cosmos must be several times that. I had a few hiccups making the switch to another server, but got it running with relatively few issues. Having a good admin panel on the server helps.
I don't think anyone could have predicted the space and bandwidth requirements would become so large when these boards started appearing in the late 90s. The sheer popularity of some communities has demanded massive infrastructure - something that hosting fees (competitively priced) can't accommodate.
I also tip my hat to anyone who runs a board. Not only can it be time-consuming, it can be expensive. If not supported by advertising or corporate sponsorhip, the owner has to pay for the hosting and any bandwidth charges, software, etc. on his/her own. So donations are always welcome.