Weighting Grades based on group size
I don't see a problem with two-part grades where all instructor grades are weighted equally. Your issue seems to be that individual teammates' opinions carry less weight in the larger teams. But I don't see why that is necessarily unfair. 

My suggestion would be to calculate the grades based on the same instructor weights for everyone and then make sure, before finalizing, that people in larger or smaller groups didn't get systematically different grades.