As I understand from most user interface design laws, the larger and object is on the screen, the easier it is to access it. It seems to me, that this rule could be applied to frequently used toolbar items.

For example, take the GEdit icon layout:
Code:
[ New ] [ Open ] [ Save ] [ Print ] [ Undo ] [ Redo ] [ Cut ] [ Copy] [ Paste] [ Find ] [ Replace ]
I counted how many times I used each of these buttons during a single session. You can see that I used some buttons more often than others:
Code:
                                      *
                                      *
                                      *
                                      *
                   *                  *
                   *                  *                                           
  *       *        *                  *                                          *
  *       *        *        *         *        *                                 *        *
[ New ] [ Open ] [ Save ] [ Print ] [ Undo ] [ Redo ] [ Cut ] [ Copy] [ Paste] [ Find ] [ Replace ]
The Undo button is used the most frequently.

I use keyboard shortcuts for Copy and Paste and I can't remember using Cut once in the last 10 years. These items aren't used as often so they shouldn't hog as much screen real estate as the other programs.

Who decides which icons are more important? The computer does. By analyzing your habits, the computer is able to gradually adapt the interface to increase your productivity.