I think that's why he proposed a rule against it. One option is to cap the number of provinces allowed based on how many there were in each kd when war was declared. That way you can fill spots for people who leave, but can't grow to a 25 v 20 or 22 advantage.
Frankly it's annoying to have to go from 22 to 25 provs just because the other guys did or take the loss. Some kingdoms like to stay smaller and shouldn't be forced to grow in size.