This is also measured in MegaBytes(MB). Bandwidth is what gets used everytime a visitor views your web site. (Traffic)
When a visitor visits your site, all the IMAGES and HTML files (everything they view) gets downloaded to their hard drive in what is called "browser cache". So the bigger your website the more bandwidth will be consumed.
A popular site with popular content may be small in size but because of the amount of traffic it will consume more bandwidth. From experience we have seen Modelling Agencies, Adult Content, Estate Agents and Cellphone Content type of web sites consumes the most bandwidth.