The mouse was invented by Doug Engelbart. Engelbart was a radar technician and spent a lot of time in front of computer screens. Apple's new mouse is shaped a little like Engelbart's original one, only lower, with rounded corners. This is just about right from an ergonomic perspective.
Experts say that a larger mouse keeps the hand in a relaxed natural position.
According to research at the Cornell Human Factors Laboratory, a larger mouse keeps the arm and hand moving together. Large natural movements, rather than jerky twisted ones, help the body avoid repetitive stress injuries. Instead of pressing a button on the Pro Mouse, you press the whole mouse. Ergonomically, this is also in line with the research. Extending the finger, especially when the arm and hand are in an unnatural angle, causes the finger tendon to rub repeatedly on the carpal tunnel in the wrist.