Modern websites will allow users to change the font size if for example they have difficulty viewing the text. Older websites will often use fixed fonts which the user cannot alter, and therefore text size may vary when surfing the net depending what settings you have made on your browser.
If you use MS IE then click VIEW then TEXT SIZE and then click NORMAL. Clicking on any of the other options will change the size of the text if that is an option as described above.
Alternatively hold down CTRL and spin your mouse wheel to change text size.