The design on the right makes use of the entire available space, and is much easier to target. If you are reading this on the Web, try to target each of these buttons by starting at different parts of the screen. You'll notice the effect of Fitts's Law, and how you have to pay more attention to targeting the example on the left. The example on the right shows how a larger clickable space is easier to target. Figure 2 shows examples that illustrate how Fitts's Law can make buttons easier to click.
Most Web browsers change the mouse cursor when the mouse is over a clickable target.
This is a nice reinforcement, but the user shouldn't have to rely on that to figure out what is clickable on the screen. In both of these examples, the visuals imply that the clickable area is the entire space between the white borders. What you imply to a user should almost always be exactly what you give them. If you force your user to spend a lot of time wandering around the screen figuring out what they can click on and what they can't, they might not want to come back again. Another important area where Fitts's Law is often ignored is on search results pages.