Sigma 150-500 on D300: weight: 1910g, dimensions: 94.7x252mm
Nikon 70-200 alone: weight: 1395, dimensions: 87.0x215mm

The sigma weighs 37% more, is 9% more in diameter and 17% more in length.
In practice, you get much more reach (less quality, of course) for not much more bulk with the Sigma.