Weight comes down to build in this case, Canon and Nikons are built like bloody tanks, This is not...The Sigma 24-70 HSM is similar in size from the looks..the Sigma is actually a hair lighter, by about 30g...the Canon is 120g or so heavier than it...the Nikon is 70g heavier... So im not convinced its really all that "OMG LIGHT!"
VC doesnt add as much size and weight as most people expect i think, the 17-50 VC isnt much bigger than the 17-50 non-vc, the Sigma 17-50 OS isnt much bigger than the 18-50 f/2.8, and the difference in size and weight in the 70-200 families in Canon isnt significant
VC/IS/OS/VR systems are only a set of small diameter lenses usually near the rear of the lens, they're not very big, nor very heavy..the mechanism for the gyros and the actual motor to move the lenses around would add girth, but not much weight....
That's cool to know
so now where is that Canon 24-70mm IS... ok wait I don't use that, where's that Nikon 24-70mm VR?
The 3rd party manufacturers are really applying the pressure, by designing unique/better-equipped lenses than Nikon and especially-Canon!! This can only be a good thing. 

