In my attempts to understand where area to the power of (3/2) came from, I did some more math.... sorry
This page talks about scale and it's relationship to speed, area, mass, etc:
http://jmquetin.free.fr/tips5e.htm
It concludes, and has a small graph, showing that wing loading is propotional to the square root of weight as model scale varies.
Starting with that I did the following:
W=Weight A=Area r=scale factor
prop = "is propotional to" or "varies with"
W/A prop W^(1/2) (wing loading is proportional to square root of weight)
Also, weight is proportional to the cube of scale and area is proportional to the square of scale, so we can also say that wingloading is proportional to the scale:
W/A prop r
hence
W prop Ar
So that means:
W/A prop (Ar)^(1/2)
square both sides
W^2/A^2 prop Ar
so
W^2 / A^3 prop r
Let's decide we don't like look of that, so take the square root of both sides:
W / A^(3/2) prop r^(1/2)
The cubic wing loading is proportional to the square root of the scale ratio.
So perhaps who ever came up with cubic wing loading liked the fact that they had a number that looked something wing loading ad was related to the scale factor of the plane.
But... I don't how you'd go from that to a conclusion that planes of differing scale will fly equally well if the cubic wing loading is that same. Actually, I come to the opposite conclusion. One shouldn't look for a constant cubic loading at all... one should look for cubic wing loading to increase with the square of the scale factor.
... time do get back to making a living.