Expanding Polynomials over the rationals

Let $F(x,y)$ be a polynomial over the rationals. We show that if $F$ is not an expander (over the rationals) then it has a special multiplicative or additive form. For example if $F$ is a homogeneous non-expander polynomial then $F(x,y)=c(x+ay{)}^{\alpha }$ or $F(x,y)=c(xy{)}^{\alpha }\mathrm{.}$ This is an extension of an earlier result of Elekes and R'onyai who described the structure of two-variate polynomials which are not expanders over the reals.