increasing the size of the rear sway will increase the wheel rate of the side of the car that is loaded in a turn.

meaning it will make as if you have higher spring rate springs in there.

so if the initial setup (stock) was for understeer, larger front sway bar and the rear sway bar size was increased then it would reduce the understeer some what.

going from 18mm to 22mm is a large jump and given that the front is also 22mm (from reading this thread) I would "guess" that the understeering is probably nearly gone.

what happens when this is done is that people tend to make the car oversteer a lot more, where they used to understeer.