<Forgive me, I don't know how to insert an image within the text, so it appears at the bottom>
Fitts's Law
See the Fitts formula at the bottom, where:
T is the average time taken to complete the movement.
a represents the start/stop time of the device (intercept) and
b stands for the inherent speed of the device (slope).
These constants can be determined experimentally by fitting a straight line to measured data.
D is the distance from the starting point to the center of the target.
W is the width of the target measured along the axis of motion.
In the case, where we are simply reducing the size of the iPad by 19% to create an "iPad mini":
a and b remain constant;
D and W change proportionally, e.g. the target size is reduced by 19% and the distance to the target is also reduced by 19%.
Since the formula divides D by W the resultant T will not change.