You look at your computer and see that 2 gigabytes of space are quickly used by adding new programs and files and you feel the need to add more hard drive space. Many people relate this to the internet and look at 25 megabytes as very inadequate. Consequently, the ISP is focused mainly on making sure that subscribers can access the internet without receiving a busy signal. When offering a business web-site, ISP's offer the very basics at inflated prices.
Every hard drive has it's limitations.
They know that most people only use 3 - 5 megabytes of space and that most people do not promote their websites well enough to draw a lot of traffic. If every website hosted on these services would take them up on the offer of unlimited space and actually used it, then these companies would soon rethink their policies and institute limits. These companies also generally offer no support and poor service. Data transfer is how much information is passed from the server to those accessing the site. Basically it is how many visitors to your web-site.