Dave, I'll try to explain.
I think one problem might be that it is not really clear what each person considers to be a significant difference. I think if scales show let's say for example a 10g difference, then yes, it's not a significant differnce.
But to be honest I don't think it's that important what the scales show. What's to me is important is how the instruments feel, and the Borgani felt much heavier than the Conn. What I mean by "much"? It just felt heavier when picking each of them, plus when playing them without a strap, the Borgani was a lot more tiring to play for a while. So my opinion is that, if the scales will later show, just for example, that it's a 1/4 pound difference, then I'd consider that difference significant.
Of course that can be partly from the weight balance of the instrument and not just the weight, but I still think how they feel is the significant thing.
But since you are interested in actual measuremements then no problem. I can't weight the Borgani because my friend who owns it is in USA for about six more months. But I still have the Conn. The best way I could weight it, with my scales which are suppsed to be accurate to the closest 0.1kg, it was 1.1kg. That's approx 2.45 pounds (could be a little more or less). I could also tell you that it's definitely the lightest soprano I've ever seen, and when I first held it I was suprised by how light it was.
So if some of your sopranos weight about 3.5 pounds, some possibly more, then I consider that a very significant weight difference from this old Conn. But that Borgani felt a little heavier than any other modern soprano I could remember holding. Nothing especailly accurate, but I just don't remember ever having that feeling of such a heavy soprano from any other insrument. This is why I noticed it. So my guess is it weights even a little more than other modern sopranos. Comapred with that Conn, that is a very significant difference. Which only show what was obvious when simply holding the saxophones and playing them.....
;-)
As to what causes the difference, I think it's a lot of little things. A high F# key, possibly a heavier material for body and keys, etc. Some of the keys and linkages of the Borgani looked especially "beefy", bigger and thicker than the Conn's, etc. which adds to the weight. Seems that all those things add up in the end to a significant difference.