Quitty,
For a solid body, the mass increases eight times when the dimensions increase twice.
For a model airplane, having so many cavities, that proportion should not apply, I believe.
I have compared two electrical RTF models sold by Tower, in order to obtain a more practical proportion.
They come with radio and motor, just RTF.
http://www3.towerhobbies.com/cgi-bin...&I=LXXJS8&P=RF
http://www3.towerhobbies.com/cgi-bin...=LXSGY1**&P=RF
Wingspans = 68.5” / 44” = 1.55 scale factor
Lengths = 56” / 36.5” = 1.53 scale factor
Weights (minimum) = 112 oz / 25 oz = 4.48 scale factor
Weights (maximum) = 136 oz / 28 oz = 4.85 scale factor
Sorry, I couldn’t find any RTF models with scale factor of two for dimensions.