I am not sure about this but don't they come in pairs? If one gets bigger the other gets smaller. The diff. case has a certain spacing so the two gears must fit each other. If they do not match they will run in a bind and wear out very fast. They also will create increased drag on one another and you will have a performance loss.
I am speaking from a real car point of view. Ring and pinion gears are always a matched set. I believe it has to be the same for the RC Cars.