A headline of a story is large for a reason—to get the readers’ attention and to emphasize the main idea.
Fonts are measured in points. Point size of a font is the measurement of the height of a font that you are going to use within a design (72 pts. per inch). Utilizing the font size appropriately can help communicate your idea appropriately.
For example, your font size can help emphasize a sentiment or hide a photo credit. Hint: When designing any printed material and want people to read it comfortably, don’t go any smaller than 9pt. (Again, rules are made to be broken…)
Fonts are measured differently for web design. Here is a great site for more info about website type setting: http://typecast.com/blog/a-more-modern-scale-for-web-typography