As for any interactive element, the sizes of the visible target and the touch target for a button can be different. More often than not, they should be.

 

When the target is a button, the bigger, the better—but only up to a point. […] If buttons get too big, users start having problems perceiving them as clickable buttons — and sometimes even discerning them as discrete elements.

 

…current touchscreens sense only the geometric center of a user’s contact patch [where the finger touches the screen]. … Because a device uses only the centroid to determine what a user is tapping, the extent of the contact patch doesn’t matter.

Very good read about touch-interaction misconceptions, via UXmatters.