relationship between nominal voltage and cross-sectional area
Power distributors plan their lines for maximum profit. They can allow more losses if that keeps the lifetime cost smaller and the voltage variation norms still can be fulfilled. The cost estimation is stochastic - it takes the probablity of accidents and storms into the account, too.

If you have noticed some dependence between voltage and cross-sectional area, it is the result of forementioned  for lifetime calculations, not only because the current is different at different voltage levels.