Thats my point. Just because something is bigger in volume does not mean it is bigger in weight and vice versa..
Anyway, I will try the line..if its too heavy, its too heavy.
If that really is your point Rob, I'm very worried
We are not talking about densities here. it is the weight of line relative to the length
Again, in case I was not clear enough second time around I'll use DH lines, you may find the comparison easier to grasp.
Go look up weights for Rio Power Spey in a 10/11, then do the same for the Uni Spey and AFS in the same size. In the order I listed them, you will see a decrease in head weight as the length decreases, they all still work a 10/11 rod at the lengths they are designed to be cast.
From memory a power Spey 10/11 weighs 825 grains at 71', a 10/11 AFS weighs 640 grains. If you stick a 640 grain (8/9) power Spey on your 10/11 rod, it will feel far too light likewise, if you stick a 10/11 (640 grain)AFS on your 8/9 rated rod it will feel too heavy.
If this still makes no sense, get Cas to read the whole thread and maybe she'll explain it to you
Cheers
Ben