In a nutshell, it is because the overwhelming majority of web site developers think browser defaults are "too big".Many of the people who train site developers agree. Example: Owen Briggs, who wrote ". . . . most browsers default to a text size that I have to back up to the kitchen to read."
Instead of doing the right thing before doing design work, setting their own browser default to suit their own preferences, they presume you didn't change yours either, leaving it "too big" for you too! So, they rudely impose this judgement upon you by overriding your default within their page designs, regardless what your default actually is.
They don't. They can't.
Most probably know what logical size it probably was when your browser was installed, while quite likely not being aware that that size is a logical and not a physical size, or how little can be the correlation between logical size and physical size, much less correlation between physical size and user needs.
The default size used to be physically quite large when the most common resolution was the Windows 95® default of 640x480, and continued to be rather large when, after the release of Windows XP®, the default and most common resolution had progressed to 800x600.
However, the median and most common resolution has long since progressed beyond 1024x768, while the spread between lowest (palmtops @ 100 wide) and highest (3840 wide or more) has grown much wider. It is long since reasonable to assume anything about the size of the default. At the median resolution on today's two most common desktop display sizes, 19" & 23" respectively, the default is typically somewhere between 20% & 60% larger than typical magazine copy and front page newsprint, while PC displays are typically viewed from approximately 50% farther away than magazines and newspapers.
In spite of this evolution to text little different in apparent size from newsprint, and the superiority of printed letterforms over display letterforms, most designers continue to impose the arrogant assumptions that:
How about I visit your house whenever you are watching TV? Whenever you change the channel I adjust the volume down by 42%, regardless whether you had it up to max or down to a whisper or anything in between. Would you want me to do that? Is there a significant difference here?
I can't read their minds, but here are some reasons they give to justify smaller page text size, as well as a few reasons I haven't seen expressed:
The paragraphs on this page use your default size, as well as your default text style. For a good place to learn how to make changes to your default settings, visit Syntactic's setting up your browser page or Steven Poley's what every browser user should know page.
Whether and to what extent you can depends on your choice of browser, the site, your ability to understand menus, and/or your ability to construct a user stylesheet. Which type of change you choose to make will depend whether you wish each change you make to remain in place as you open and close the browser or move among web pages.
Since what web sites are doing usually amounts to overriding your default, changing the default can be counterproductive or ineffective. If you make the default larger, then on sites that don't shrink text you'll no doubt find text is too big. Other sites set absolute sizes, which are not based upon your default. Hotkey zoom is the simplest to apply, if available, but tends to make you go back to the well time and again to change size as you go from page to page. So, the best available tool is probably the user stylesheet. Unfortunately, your browser probably doesn't make configuring one particularly easy, while typical site styles are so complicated that even site designers have trouble determining how to override styles on other sites.
Try this, if you like serif text, courtesy of Karsten M. Self. If you like sans-serif text, you can use the same file. Simply use a text editor and substitute your favorite font for "Garamond" following the word "BODY". Elsewhere in the file, replace all instances of "serif" with "sans-serif". Do yourself a favor, and don't make Verdana your default. It really only looks and sizes well at sizes smaller than average. Customizing Mozilla provides examples of customizing you can do in your stylesheet. Jesse Ruderman also has a helpful user css page.
In the retail business there is a universal2 management policy:
The customer is always right.
How different is the world of web design! Those that pay the designers are considered the customers, and the needs of the real customers, those that use the web, are usually treated as though their needs didn't matter at all.
1-There is good reason for toolbar fonts to be smaller than page text. Users quickly become familiar with menu items and what they represent, becoming routine targets to click to achieve quick action. Web page content isn't at all like that, mostly being unfamiliar and more voluminous, generally requiring some attention and focus to assimilate. Larger than menu page text facilitates this, while smaller menu fonts save some space to allot to the content, and some CPU cycles, by needing to draw to smaller screen areas, thus making the OS seem faster than it otherwise might.
2-Some businesses have an alternative philosophy.