Typically, bikes used in the Tour De France are not usually exceptionally light compared to what is available.
Professional riders ride so many more racing miles in tightly knit groups that crashing is not an unusual occurance, and the bikes need to be able to stand up to this.
Specialist hill climbers, or perhaps team leaders may change their machines during the stage to switch to a very light machine, but the biggest differance will probably be in the wheels and tyres.
You can’t save all that much in frame weight, a Reynold 531 frame does not weigh all that much more than a carbon fiber frame, perhaps there’s as much as 1and a half pounds differance, it is very surprising when you actually pick up two frames with no components attatched.
The biggest differances come from the stiffness and springiness of the frame, and also the weight distribution, as light aluminium and carbon fibre farmes tend to be very light at the back end, and when this is combined with their stiffness, can make for a faster accelerating machine, or for less enrgy to be wasted in flexing the frame when mountain climbing.
The really lightweight machines used to be time triallist machines, with 16 spoke wheels, ultra short wheelbases, and titamium sprockets etc.
These things were too fragile to be used in the rough and tumble of a bunch race, where you usually don’t get the chance to see and avoid poor features in road surfaces.
Times have changed in time trialling and now aerodynamics matters more, they still have the silly short wheelbases though which is why the riders have so much trouble getting around tight bends, and the disc wheels and ultra stiff farmes also add to the problem.
The typical Tour De France or any proffesional rider’s machine thends to have a more relaxed geometry, the fram angles at the front are usually less steep than the rear frame angles, this makes for a machine that is more stable to ride and less twitchy over longer rides and hence less mentally tiring.
Geometry is what really makes the differance, but try getting that across to a largely cycle racing illiterate public, which is pretty much what the US is.
This is not an insult, but things like this are not easy to explain to a population that does not really have a cycle racing culture like Europe has, most folk in the US watching Lance Armstrong in the US can readily understand weight, and will understand what a mountain is, but things like handling, flex and geometry are not too good to put into the 30 second tv news slot.