I too would be upset with the size difference more than the plate issue. Sure, the fewer connection points the better, but that is in an ideal world. I would rather have the larger cable over anything else. So don't worry about the plates, they will look nice and clean, but the points you could argue with the builder (spin it to your advantage) is that 1) the gauge is smaller than you NEED, AND 2) they just added more connection points which will cause a decrease in signal quality and introduce signal noise because it is no longer one solid cable from amp/receiver to the speaker.
Again, this is to try to push them to fix the problem. Will it be earth shattering, no. Will you notice a difference in sound quality, probably not (unless you are using testing equipment). I too hate it when contractors (or anyone for that matter) try to cut corners, or don't do an accurate job and try to cover it up later.
Good luck!
