The size of the tail has a different reason. It has to do with the controllability of an airplane during a one-sided thrust loss (engine failure case). A longer fuselage needs a greater turning moment, increasing directional stability. Now the same plane with a shorter fuselage has to compensate this loss in stability. That is done with the larger tail surface. On the A321-A319 a one-sized tail is sufficient, but on the A318 it will be heightened by several feet.
Just a basic explanation.