Think of stretching a rubber band; if you make it significantly thinner (radius squared impact) while only slightly stretching its length, the total material volume actually shrinks.

Square the small (radius), multiply by the tall (height) to find the change for all.

Given volume $V = \pi r^2 h$. If radius $r$ changes by factor $(1 - 0.20) = 0.8$ and height $h$ changes by factor $(1 + 0.20) = 1.2$, the new volume $V' = \pi (0.8r)^2 (1.2h) = \pi (0.64r^2)(1.2h) = 0.768 V$. The change is $1 - 0.768 = 0.232$, which corresponds to a 23.2% reduction.

Whenever a dimension is squared in a formula, any percentage change applied to it must also be squared in the net effect calculation.