It all depends on who you ask and what they are doing.
This is a webhosting forum, mainly visited by web hosts, and so due to the nature of what we do our sites are often more complicated and larger simply because of everything we need to cram in (e.g. a hosts website might include the content, billing system, support system, knowledgebase, forum software etc...). This site would include the advertising system, the helpdesk and the forum software - plus all the databases to power them.
Most of the 'small business' and 'personal sites' I host typically use 0-100mb, a few of the larger ones that have been going a couple of years are over the 1GB mark, but, that's still a very small percentage. On the other hand, an 'image host' will run into the GB's and potentially across servers.
Taking wild figures doesn't really make much sense IMO. You'd be better to focus on a certain thing - "how much space do web forums use", "how much space does an average blog use" etc... as those figures are much more useful in the real world
