In GUIs, there are clickable objects that have a difference between the motor and visual widths. For example, when looking at an item on a navigation bar, users think that the text length (the visual width) means the motor width. However, when a cursor hovers over the item, the cursor shape changes or the item is highlighted, and then users understand that the actual motor width differs from the visual width. In this study, we focus on the difference between the motor and visual widths and investigate how the difference affects user performance. Experimental results showed that 1) users aim at the motor width, 2) the reaction time is a U-shaped function whose optimal point is located where the motor and visual widths are the same, and 3) the movement time depends on the motor width. We also analyze existing GUIs and discuss the implications.