Do you have gzip enabled? That will increase CPU usage but decrease bandwidth. In the ACP on the general tab click server settings link found on the lower left.
Most hosting plans will include Awstats which can give you some detailed log analysis, see if it's available. One thing it will list is bandwidth by IP, you'll probably find a few IP's listed at the top that are are not from residential ISP's. These would be bots and there is very little you can do within phpBB to address this. They need to be blocked before they request a page and that is not something easy to accomplish because the IP's will change.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison