A few theories about this, that I've thought of, is
>This is the only city to host a Pokémon League in the games. However, besides the league itself and a Pokémon Center, there are no other buildings in Ever Grande City, residential or otherwise.
>In Omega Ruby and Alpha Sapphire, the Pokémon League is displayed as a separate location on the AreaNav's map and has a separate location index. However, it is still considered part of Ever Grande City, as suggested in dialogue with Sidney at the Battle Maison.
-- Bulabapedia on Ever Grande City
Thus, it seems to me, that Ever Grande City is not a city because it has lots of people. Instead, it is so because it hosts the Pokemon League and proves as a home in some ways to the most powerful trainers in Hoenn, the Elite Four and the Champion (although his physical house is in Mossdeep City), and it symbolises the power of all of Hoenn.
Also, given that that the Pokemon League is a part of the city itself, it automatically makes the city much bigger, and makes the Victory Road a bridge/ tunnel of sorts from one end to another.
If the word "Grande" was to be seen as a status of existence over size, we see very clearly, why, Ever Grande, despite not housing Hoenn's population, still is a major part of it. It has been a place of power since the beginning, and thanks to the player saving the Earth (surprise, surprise!), will probably continue to do the same for several years to come.
Besides, the Japanese and Chinese translation mean
>Possibly from 最優 saiyū (best/excellent). Contains 彩 sai (color). [Japanese]
>Means "Colorful secluded city" or "Colorful serene city" [Chinese]