Paper typography uses many different font sizes, often for some definite purpose. Bill board advertising has no control over viewing distance. The advertiser must pare his message down to a few large font words for those on the other side of the street, and have more information, in a smaller font, for those up close. Newspapers have big headlines, intended to be read while the paper is on the shelf at the newsagents, and small type, which tells the stories and is read at home in an arm chair. Newspapers also have several distinct stories on each physical page. They need headlines to help with framing, that is, dividing up the physical page, and not just heading the stories.

Paper actually exists, as a physical substance. So it is natural to economise, by using small fonts. This works because paper is a high resolution medium. If you take a magnifying glass to the small print, you discover what it says, not that there are too few pixels to form letters.

Little of this carries over to the web

Typically you sit on a chair in front of your screen. The viewing distance is fixed, and you have it set with the standard font as small as is comfortable, to make the most use of limited screen area. Slightly larger fonts help to mark out titles and section headings, but big text is just wasted space. Hypertext places each article in its own window, the headlines are not breaking a physical page into logical units.

Small fonts crumble into illegibility. What could the website possibly be economising on? A change of font size marks the text as a separate logical unit. If a page is overfull, pull out a separate logical unit, and put it at the end of a link on a page of its own