Yeah, it's that pesky physical law of aperture-opening-to-focal-length ratio, along with the fact that the flux density of incoming light is fixed for any given scene that gets in the way of designing/making lighter lenses.
The longer and brighter your zoom, the larger the front-most elements must be. The only hope is to have smaller sensors (but sacrifice high ISO performance, small DOFs, and lens optical quality must be much better due to larger crop/magnification factors), or somehow use a material that is less dense than the current glasses used for the high-end lenses, or come up with a simpler lens design that doesn't need as many glass elements. For the lens barrel, I think they would have scrimped on as much as the weight as possible by now, without resorting to exotic metal alloys (magnesium-alloy?) or super-tough engineering plastics that will drive costs up to the moon.
