Considering only the dimensions, the second generation iPad managed a meager trimmings of 4 mm, 2 mm and 4.6 mm in length, width and thickness for a total of 10.6 mm.
That pales in comparison to a total of 18.9 mm in combined length + width + thickness savings, in the current iPad Air.
So your elusive measure of "perceived bulkiness" falls short of explaining how iPad Air is not the greatest leap in terms of portability over its previous version, over all of the other deltas of iPad generations.
The "combined length+width+height" metric you've invented is highly dubious. A 1mm reduction in a dimension means much more if it's the shortest dimension than if it's the longest.
Considering only the dimensions, the second generation iPad managed a meager trimmings of 4 mm, 2 mm and 4.6 mm in length, width and thickness for a total of 10.6 mm.
That pales in comparison to a total of 18.9 mm in combined length + width + thickness savings, in the current iPad Air.
So your elusive measure of "perceived bulkiness" falls short of explaining how iPad Air is not the greatest leap in terms of portability over its previous version, over all of the other deltas of iPad generations.