Into the Lion's den I jump...
Hilary may be onto something - it took me a bit to put the pieces together.
My sense is that 1 1/8 top tubes tend to transmit roadshock differently than 1" top tubes. The larger tube seems to smoothen out the ride a bit. But the kicker is that if road shock is transmitted differently, the feedback one gets from the front wheel is different too. I think that this can often seem to provide a feeling of greater stability.
I have to say that I really like oversize tubing - to me it feels different than skinny tubes. Also, there is now more ability to use oversize tubes in thin gauges due to the advent of the air hardened steels such as 853.
And that doesn't mean I don't like skinny steel - I have bikes of both types that I love! But they are different...
As my engineer friends have taught me, when it comes to torsional rigidity, it is only tube diameter that matters - not tube gauge! So the ability to use larger tube diameters helps the builder to design the optimal frame for a given rider when such a tube is helpful.
As has been said before, it isn't the material, its really about matching the material (and by that I also mean tube gauge and diameter) to the rider and the purpose at hand.
In fact, I've been on modern aluminum bikes that behave more like nice steel (to me anyway) than some steel bikes!
Mike Kone in Boulder CO