QAVION - Are you sure you've got it the right way round.
Add plugs to a fuselage and the length of arm gets longer. As with any 'see-saw' the amount of pressure required to produce the required effect will REDUCE as the length of arm gets longer. i.e. To produce the required effect on an A321 will take less elevator deflection than an A319.
The other example above clearly illustrates this....when Boeing shrunk the 747 to produce the 747SP they had to increase the size of the tail in order to produce greater forces to compensate for the length of arm.
In trim