I'm not saying that your fingers are doing any extra work at all. I'm saying that the position of the sensor gives you better control when it is mounted farther forward on the mouse, regardless of which part of your arm or hand is being used to do the work.

Think of your entire arm, from the elbow all the way up to the fingertips, as a single large lever.

Now imagine that lever sweeping back and forth, pivoting at the elbow. Or the wrist. Or the shoulder. Or wherever you think it should be pivoting based on what you think are the best ergonomics.

Now imagine the distance that the sensor sweeps, depending on whether the sensor is mounted at the back of the mouse or at the front of the mouse.

In a given arc of movement of that lever, you get more sensor position travel if the sensor is at the farthest point from the pivot point of that lever. The farther back you move that sensor point, the less travel, the less sensitivity, and the more your arm (or wrist or elbow or shoulder or whatever) has to work get the movement to register.