I have some experience with mpmetis from METIS. It is pretty good software which offers unstructured mesh grid partitioning. But obtained results always minimize edgecuts or total communication volume. So splitting simple mesh grid into 2 parts gives me about 50 to 50% of mesh nodes in each part.
I am looking for software (or better C++ library) which gives me possibility to split mesh into unequal parts - for example 10 to 90% of nodes in each part. It will be nice if I can decide about divide ratios.