if you cut a section through a standard disk, you would have two parallel lines representing the outer and inner faces of the rotor.
for a slotted rotor, the lines would be interrupted by grooves representing the slots. if you measure the distance across a straight line versus one that jogs in and out - even once - you would see that the straight line (shortest distance between two points) has less length (corresponding to area when converted to three dimensions) than the line that jogs in and out.