You might want to rephrase your question in a more general way (e.g. "What features are needed in a tripod to make it suitable for long exposures?")
It's not as simple as more weight = better for long exposures. For that, what matters is rigidity and stability. So, larger diameter sections (a large diameter tube is more rigid than a small diameter one as stresses are primarily on the surface.), which use more material than smaller diameter, and more material = heavier. I'd also suggest you ditch the center column, so more material for a given camera height, and more material = heavier. The bright side here is that, without a center column, the legs are longer, making the feet wider apart, making the tripod more stable. Oh, and no center column means you'll probably need an 'L plate' for on-tripod portrait orientation - and those weigh a bit more than a standard plate.
About the only thing you can do for rigidity that won't cost you weight is use 3 sections rather than 4 or 5 - but you pay in a larger collapsed height, so more awkward to move around.
There's also a weight/cost tradeoff to be made. Switching to an equivalent CF tripod will save, hmmm... 20% of the weight of an equivalent alu tripod for, hmmm... around double the cost. CF has the advantage of also being less resonant than alu, so vibrations from shutter release are better damped.