Every time one of your web pages is viewed by someone on the internet, the size of that page goes towards your bandwidth usage. Below is an example of how many times a page would have to be viewed to reach 100 GB of transfer

30 KB pages = 104,857,600 / 30 = 3,495,253 views
40 KB pages = 104,857,600 / 40 = 2,621,440 views
50 KB pages = 104,857,600 / 50 = 2,097,152 views
60 KB pages = 104,857,600 / 60 = 1,747,626 views