Well, this is explained on their policy page. Since most of my files are stored on another server, I'm not really concerned. I find it wierd that sites with banners are set to a 6GIG limit though. The banners are usually stored on other servers.
WestHost's customers are privileged to be offered unlimited traffic. Unlimited traffic applies to standard web documents (JPEG, GIF and HTML). All other types of files such as (ZIP, MP3, etc.) are subject to 3 GIG of transfer a month. Sites containing paid advertisements, such as banner ads are allowed 6 GIG of transfer per month. Any customer who exceeds this limit will be notified by WestHost to resolve the situation. Extra bandwidth is $15 per GIG.