What plane are you flying? Because the answer is the standard "it depends".

Aerobatic planes frequently have rudders larger at the bottom than at the top in order to move the the rudder forces closer to the center line of the aircraft. One of the reasons this is done is to reduce the tendency for rudder application to cause some yaw-roll cross coupling.

It's not the rudder's relationship to the stab that causes this phenomenon but the rudder's relationship to the aircraft center line.

If most of the rudder is on one side of the CL, deflection of the rudder creates the intended yawing moment but since the force is all on one side of the CL, it will also try to rotate the aircraft. Think of it as an aileron on only one side. No force on the opposite side of the fuse so it causes some roll.

If the rudder area were equal on both sides of the CL, the yaw forces would be equal above and below the center line so there would be no tendency for rudder to cross couple to roll. Think of it like an elevator, except it's vertical. The forces are balanced so there's no roll.

Since the rudder usually doesn't extend past the bottom of the fuse, the way to move the rudder force towards the center line (and thereby reduce yaw-roll coupling) is to reduce the area at the tip of the rudder and increase the rudder area near the center line of the fuselage. Thus the generally triangular rudder shape you describe.

Dave