Think you'll find that the effectiveness of the vertical stabiliser is reduced because of the 'shielding' effect the larger fuselage has, i.e airflow is reduced across the lower (widest chord) area of the vert stab and this in turn decreases longitudinal stability. This reduced area of effectivity is compensated for by the addition of the 'winglets/fences' on the tips of the horizontal stab.
You'll see similar devices on the NASA 747 that carries the Space Shuttle.
