Funny that I ran by a brand new set of Magnum tubular arms at the Nats and picked them up in my hands to trigger my mental scale as to how light they are since mine have been on the car since when they first fell into market and they were light indeed.
The Tubular arms are made out of Chrome-moly and for that, they may be able to built them utilizing a thinner wall with a few gussets to make 'em stronger then the stockers.
The stockers are light as well, but just a tad more then the Tubulars.

Not sure if there is a difference from A to B UCA's or if small bolt pattern vs big bolt patterns is the smoking gun, but the extra cam and caster that the tubulars give you is what really sold me on top of the weight.
On another note, there might be a new light weight Alloy Dana 60 rear that's coming into market and I saw the prototype at the Nats. Beautiful and Goliath looking? Yes.
Suspect I am? yes, but it was neat indeed with all the strong points in all the right places and supposedly 50-80 Lbs lighter. Reminds me of the magic Mopar Performance 8-3/4 Alloy Pumpkin that came out years back. We'll see because as we all know, that Eureka Vacuum cleaner salesman always had all the answers and magic products at the door.
I'll try and post a flick later.