[Andrew Musau]

I do not think that you can specify different scales if using the by() option. For other graph types, you could have plotted multiple graphs, but twoway does not allow for multiple contour plots. Therefore, you can either plot size(log) separately or re-scale the output to match the rest and then add a note that the graph is re-scaled by some given factor.

[Damian Tojeiro]

Thanks a lot for your help Andrew