I have an idea that I think solves the question of what does World Class mean, it is the maximum Population that you can have on that World, not the number of tiles available or potentially available.
To be more precise, it is the largest number that your population will grow to on that World normally. You can put more population there with Colony Transports and Troop ships, but you get additional penalties to Approval (and possibly to revolt risk, unrest, etc.) by doing so.
This could explain why the World Class never seems to change when we add additional usuable tiles by terraforming and other events, ideologies, etc.
I am not sure if techs or other things change the population limit for your Race, etc., if that affects the World Class, but I doubt it. I suspect that population limit is almost the same thing as World Class, but there is some difference in their actual meaning.