These over-sized downtubes seem like a styling trend.
Several frame makers/providers have recently made a larger downtube part of their design; some very subtly and some not so subtly.
Aesthetically, I prefer the traditional designs which have all same size (or close enough so one wouldn't notice it) tubing.
Techone's bike, which he describes as utilizing Dedacciai OS tubing, has that classic look, to me, despite the OS description.
I have not seen a Bianchi Pista in real life.
Mine should arrive at my lbs tomorrow.
From the pictures, though, it seems to have a gradual center bulge (and probably a flattening viewed from the front or rear) in the downtube (oval?).
A person could look at it and not see it, I think.
The recent KHS and IRO frames seem to have a slightly larger downtube; and, again, from photos.
In all three cases, it looks like a styling thing more than an engineering thing.
I'd like to see some engineering data regarding all of this.