It is entirely dependant on airplane size. Without trying to get too far into it, it has something to do with Reynold's numbers, which has to do with airspeed and size and such, which basically takes into accout the size of things and the way they react to gravity, air pressure, etc. If you want to delve into the theory, you can go to http://home.earthlink.net/~x-plane/F...-Reynolds.html or type "reynolds number" in a search engine...
To give you an idea, the ideal wing loading for a 30" 3D foamy is somewhere around 5 ounces/sq.ft, while a very light wing loading for a 40%er is around 30-32 ounces/sq.ft. In the mid range would be a 120ish size plane, where optimal wing loadings are floating around 20 ounces/sq.ft. I have a 50cc plane that weighs 15.2 pounds with 1450 squares for a 24 ounce wingloading and flies like a kite...

BTW, did you know that a high performance full scale aircraft, such as the Yak 54, has over a 15 POUND/sq.ft wing loading?