There is no perfect way do determine the exact boundaries of a city WHS. And while there are cities like Quito that seem to have no boundaries at all or like Samarkand that include huge and also some less interesting parts of a city. On the other hand there is at least one city on the list with absurdly narrow boundaries: Paris.
I think even most WH travellers are unaware how little of the centre of Paris is included into the core zone while a buffer zone doesn’t even seem to exist. It includes naturally the most iconic sites like the Ile de la Cité, the Eiffel Tower and the Louvre and seems beyond that mainly interested in the buildings from the world exhibitions: the Trocadero and the Palais de Tokyo. Both of them are not very attractive if you view them closely and in a surprisingly bad state. On the other hand it excludes several iconic sights that border directly the current core zone. Among them the Champs-Elysées, the Palais Royal, the Chatelet and the magnificent St. Julien le Pauvre. It also excludes many of the unique sites just a block or two further away from the Seine: The Palais Garnier, possible the most fairy tale theatre in the world, the Arc de Triomphe, St. Eustache, the Place des Vosges, the Jardin de Luxembourg, the Pantheon, Saint-Germain-des-Prés, the oldest church of Paris. It doesn’t either include the old quarters that escaped the Hausmannian changes of the city like the Marais and the Quartier Latin.
It would be an easy extension to include all these sites who are part of the very historic core of the city by extending the borders just a few blocks. I am sure nobody would even discuss if this were legitimate.
Beyond this the current WHS does neither include some famous site which are a little further away from the Seine like the Montmartre area with the iconic Sacré Coeur or the Cathedral St. Denis with its magnificent royal tombs. Those two should be easy extensions as well or even independent WHS if the city and the country took a real interest in promoting them. My impression is that nobody cares because Paris is already famous and full of tourists and therefore it is unlikely anything will ever change here. The fact remains that this is possibly the most understated core zone on the entire list.