Additionally, I should address the logical fallacy you have tripped over:
If the tail arm is increased, then
(a) aircraft more stable
(b) smaller tail lift to trim
does not in any way imply that
smaller tail lift to trim makes the aircraft more stable.
To illustrate, let us suppose a different logical relationship:
The more money I earn, then
(a) the happier I am and
(b) the more tax I pay.
If I were to follow the logic you wish to use, that would mean that (b) implies (a) and therefore:
the more tax I pay, the happier I am
and that I could become happier if my tax rate were increased.
I think it safe to say that paying more tax does not, of itself, make me happier.
Simply, if X => (a) and X => (b) that tells us NOTHING about the causal relationship between (a) and (b)