I don't have the math to explain this with the correct terminology, but trust me, it happens automatically.
The fender is basically an incomplete torus:
So in this diagram you see there's two diameters; applied to the 63 mm 700c fenders, the 700c diameter is drawn in blue, the 63 mm diameter is drawn in red. If you stretch out the former, the latter gets narrower; if you stretch out the latter, the former gets smaller. Look at the two before and after photos again, and you'll see the differences.
I see. So if I follow your math logic correctly, if I try to widen my current 26" fenders, they will end up being less than 26" in overall diameter? So not a good idea? Perhaps I should just spread the area near the fork that might rub?