size. Larger wire (higher gauge) can handle more current, so splicing a higher gauge wire to a smaller gauge wire will limit the current flowing through the wire to whatever the size of the smaller wire is. So as a good rule of thumb is it is preferrable to splice the same size wire to each other.
However, I believe what Woody was referring to was that in this case, (injector clip splicing) the current flowing through the large wire is still not more than what the smaller wire can handle so it won't affect the performance of the injector.