If the urban foxes are bigger, then the obvious answer would be food. Urban environments offer easy food, higher calorific foods (pizza, kebab etc). Then you can also add that larger males would get choice breeding options, so t he next generation would carry those genes. That makes sense but with cats I can see no reason why dwarfism would occur. There’s no natural or artificial reason to explain it. Plenty of food, no competition, no hunting/culling of the larger members of the population (if it exists). With respect, it makes zero sense!