If my village forms a union with your village and both our villages have 1000 inhabitants at the time I don't want your village to be able to dictate our common policy just because you have more children or more people died in my village 20 years from now. Thus when we are forming a union we stipulate that we have equal voting rights.

It's going to happen in EU in some form as well (assuming EU goes into closer integration direction) because there is no way small countries accept closer union without a mechanism similar to electoral college.