Designers have been using contrast for centuries. In early European hand-lettered manuscripts, for example, there were no paragraph delineations. The scribes inserted large initial letters to indicate the beginning of the text and to separate important sections within the text. There was no chance that the reader would miss these visual signals. Using size for contrast organizes information by importance.
Size is a common way to establish dominance on the page.
It creates a focal point and indicates importance. The greater the contrast in size, the more effectively it draws the eye and communicates significance. Contrasting size is often used to establish a heirarchy of information. The most important information can be made to stand out by making it much larger than its surrounding text. The less important information is then set in smaller sizes, decreasing with the level of importance.