There are competitions that require that a model plane be a scale of the original. It can be 1/4, 1/5, 1/6, 33%, 40%, etc. To be considered legal for competition the "scale" aircraft must be within some high percentage of correlation to the original - something like "within 5% or original scale". They (apparently) do this because some planes that are "flyable" in full size scale may just not work at 1/5 scale.

No, I am not a scale modeler (I know, it shows) - just sharing ...