Clickable elements must designed according to UX guidelines & be easy to hit with the finger. Besides, your users must always be able to see that these elements are clickable. More about finger-friendly design read here:
Finger-Friendly Design: Ideal Mobile Touchscreen Target Sizes - Smashing Magazine
In darts, hitting the_bulls-eye_is harder to do than hitting any other part of the dartboard. This is because the bullseye is the smallest target. This same principle can also apply to touch targets on mobile devices. Smaller touch targets are harder for users to hit than larger ones.