Originally Posted by SoSmellyAir

It is simply math. On the small end of the cassette, a single tooth makes a big difference, e.g., 12/11 = 1.0909. This means you have to increase your cadence by about 9 percent to maintain the same speed after shifting from the 11T to the 12T cog. This differential decreases as you move toward the big end of the cassette, e.g., 13/12 = 1.0833, 14/13 = 1.0769, etc. At some point it becomes pretty small, e.g., 19/18 = 1.0556, so it makes sense to have a two teeth jump. At the big end of the cassette, where one is generally climbing and thus going slower, one can tolerate a bigger, multiple teeth jump in order to have a bigger mechanical advantage to climb steeper slopes, i.e., a bail out gear. For example, the 7 speed cassette on my wife's cruiser bike has two teeth jumps throughout except the largest cog is 10 teeth larger than the next largest cog. But she never uses it.